Hello all!

Since I have such great success asking advice here, I figured I'd do it one more time.
With recent events (family coming from all over, a birthday milestone, etc.), I came into a little money in hand, and I figured I'd treat myself to a new smartphone.

My current smartphone (my first Android one, in fact- had a Nokia N79 before that which I still insist was a very capable smartphone), was a off-brand one with an MTK MT6589 chipset (i.e. QuadCore 1.2GHz, with 1GB RAM) that I bought near the end of 2013.

I plan on rooting the phone and seriously changing the appearance and all the default apps to my own preference, including distancing myself as much as possible from all sorts of pervasive always-connected to google type stuff (aside from whatsapp which is a necessity due to family not being technologically savvy enough to use something else...you can get a sense of my paranoia from a previous thread here where I asked for suggestions on permission free apps). So hardware specs are more important than purely software stuff. Also am trying to get into android development (am just into basics now, but I'd like to make games and such), if that is relevant.

My phone habits are mostly just normal phone usage with the occasional casual game like sudoku or solitaire (haven't been able to get into "proper" games on the phone, dunno if that is due to the quality of my phone or my perception that such games don't really work well on a phone), and mucking about with sensors (GPS, gyroscope, compass etc.). I wouldn't mind being able to use it for music listening as well (I jog and commute a bit, so it is useful), but I don't know how well that would work with regards to battery life (because of quick battery draining, I don't use my current phone for listening to music, instead using my old N79 for that).

Sorry for the long post! I have a budget of about $600 (although I won't be able to buy online), but I'd like to get something that I won't have to change too soon. I might've spent it on something else, but like I said, it is money in hand, not really usable online so no games or such. Might've gone with a battlestation desktop (that amount of money would probably net me something more powerful than the laptop I had help getting), but I'm not sure about my future residential stability :D.

I dunno, I was gonna buy one myself, but this weird old dude convinced me not to.
http://goo.gl/vv6zxc

You can look for the Xperia Z5 Compact, is of the few that you can found with hight specs but without giant dimensions (4,6", instead of five or more). The only bad thing with this one, is that the battery isn't removable.

Hahah! Gotta say, with the "Editing in progress" post you had first, I was expecting something else :D.

Honestly, that's because I messed up at first. I was halfway through what I was saying and accidentally sent it, meaning I had to cover my tracks in case you saw it, hence the "editing in progress", since it was the quickest thing I could think to type up to save myself from embarrassment.

To me it doesn't appear to make that much difference which Android phone (in a given price bracket) one gets, they are pretty much just clones of each other.

Quite often I buy some Samsung just because they are in all stores here with a good selection on different prices, and you have to search harder to find a non-Samsung Android phone here. Last year i bought one LG phone as a gift which has proven to be quite good, I am still using one four years old cheapo Huawei phone which I've rooted myself and replaced the battery, etc. etc.

The only thing I decided to skip a year ago (when buying that LG phone) was new Huaweis, as they had stupid UI decisions, like they had removed the separate application shortcut view containing all your applications or something like that. I suppose it was supposed to make the user interface more newbie friendly, but I found it very restrictive. No thanks. I don't know if Huawei has changed that later.

Then again, if one roots one's phone, I guess that doesn't matter.

Take a look at XDA developers forum to check there about which phones are most common in modding community and get regular updates. As you don`t really need high specs, you can go with quite cheap phone, just make sure it has all the sensors you want and gets good modding support. Usually I would recommend the 2015 Motorola Moto G, but it lacks some sensors you want.

I have a Note 4 and am quite happy with it. Otoh Note 5 and newer Samsung phones lack some features that I want like removable battery and microSD. Now my vote would go towards the LG V10.
Also you can't probably go wrong with a Nexus 6 if you don't mind the size.

Have you considered an Ubuntuphone? http://www.ubuntu.com/phone They're pretty sexy and apparently have full desktop OS capabilities or so.

Otherwise for Android you typically just get the most expensive phone. An HTC, Samsung, or whatever is the $500+ flagship is at the time.

This. It's not clear to me if you want to install a different firmware or just root and get rid of crapware, in which case most phones will probably work, so just find one with the sensors you want and a good battery.

Yeah, it is mostly just to root and get rid of crapware (and gain a little more control). I am not sure about custom firmware yet (as it might break compatibility with some apps...I have no idea). Is it something I should look into? Would it affect compatibility checking if I want to test some app I'm developing?

Ubuntuphone seems like a nice concept, but it doesn't appear to be as common as android (and it would get in the way of my android development). Plus, I'm probably a little biased against it ("This is the reason they ruined ubuntu with Unity!" :D).

It seems everyone has their own preference, probably could just go for the flagship device from any of the main companies, barring any specific hardware pros and cons specific to some device.

There's little reason to buy a flagship phone. These phones lose the most value over time and give you a lot of stuff that you don't seem to need.
I have bad experience with it custom firmware, but in theory it lets you upgrade to newer Android versions even after the phone's manufacturer stops supporting it.

I figured mostly for extending its usable life- the higher hardware specs so that there are more chances it'd run some app I'd want instead of being too old for it, or running it badly.
I wouldn't want it that some browser or game or app I wanted to try (or even make) wouldn't be usable (or would be frustratingly slow or limited) on my own phone even 2 years down the line.

I used to go with Samsung high-end devices but I got fed up with how Samsung handles (or doesn't handle) the open community and custom ROMs, so I switched to Sony. I'd suggest anything from the Sony Z series (or Z compact if you prefer smaller devices that are still powerful).