Well since 2160 isn't out yet, I'd say perimeter.
I've been curious about the earth series myself, maybe get one now and the other later

Earth 2160 for me, without a doubt. I didn't like Perimeter that much to recommend it over E2160.

Buy Earth 2160 when it will be released. It is better than Perimeter.

Is perimeter a type of deep tower defense game?
Does the teraforming in the game actually effect gameplay, or is it just for looks?

Get Earth 2150. It has terraforming, which is way better than in perimeter (and in Perimeter, it IS somewhat used in a meaningful manner, but not enough to make it a real feature)
Anyway, if you want something REALLY original, Perimeter is for you. It HAS some great features, like changing your whole army into something entirely different in a matter of seconds.
If you want something more 'normal', but still with unorthodox ideas - get Earth 2150. Trust me, it's better than 2160.

Get antaeus rising. : )

Hmm well I'll have to sit down with both demos and really figure it out, I don't like to binge buy RTS games :D
There is just something about seeing cool mechs in 2160 and they way they are rendered that makes it appealing.
But for sure I will let you all know what I decide!
Thanks

You can make heads or tails to decide. :P

i've played both earth has a more intersting story in the fact it makes sense lol and it's cheaper and you get to design you own verichles and have unique bases

Every race in E2160 is different. Lunar Corporation has modular tower-bases. Each tower is made from up to 5 different modules like Unit Production module, Vehicle Production, Research, Defense, Solar Power etc...
United Civilized States has buildings attached to core. Up to 4 buildings. Also they can build towers.
Euarasian Dynasty is a one, big base, connected with pipes.
Aliens... well... They dont have buildings. They are just one, breeding nation. Also their units production system is really unique and interesting.
And yes - Earth games has old school mechs.
Also E2160 campaign is nice and interesting, but I would recommend to finish E2150 first, because E2160 is a direct sequel.

What about a decision between E2150 and E2160? I've read in reviews that E2160 is worse, but I'm still quite puzzled as to why it loses out to its older brother. Fenixp could you give a better explanation? From what I've read 2160 gets thumbs down's because of the AI and interface, but is it something liveable with? Does 2150 also suffer from similar problems too, or is 2160 entirely isolated with these? Some of the more positive reviews mention that 2160 is more complex than 2150. Is this true, or is this insignificant to make one ignore the flaws of the game?

Well let's put it this way: In E2150, you have ammo bar, health bar, electronics bar and heat bar (and, sometimes, shields). As far as I can remember, in 2160, you had... Well... Health and shields bar :D (maybe those 2 other stats are in 2160 as well... But I don't think so)
E2150 feels better. You'd have to play both to know that
I've never noticed terraforming in 2160
HOWEVER, I did play 2160 just for a while. Several times, sure, but it never really catched me.
Hmmm...
YO!
(edit he knows that)

see i saw that in a trailer, that pretty much sold me on it, plus the graphics are beautiful even now.

Well I've decided I'm going with Earth 2160, cheaper, prettier, and I'm needing some mechs right now.
I do want Perimeter but not now, I'll just get it later, I've just been buying too many things lately and its starting to add up, but I told myself I can squeeze in $6 for Earth :D