Is there a comparison of these two games so I can decide which one I want to buy?

Mainly which is better and all that.

I am playing MM7 at the moment.

MM6 and 7 share the same engine so graphically there's not much difference. They both share the same basic mechanics game wise.

If you like MM6 then you will like MM7. Both have their fans.

In my opinion the choice would come down to how much you want to spend. MM7 is the cheaper purchase but you only get the one game whereas with MM6 you get six games for a little more money.

I have found the difficulty curve in MM6 more lenient than MM7. Maybe it's just me, but I have been finding MM7 an uphill struggle. MM6 started off quite difficult, but once I did a few run here, run there quests to level up my characters and buy some better equipment, it became less of an impossible struggle and more of an enjoyable challenge.

They are similar and use the same engine, M&MVI is bigger and more hard core, while M&MVII is more streamlined, with smaller and more boring dungeons and you can choose a Dark or Light path at a certain point in the game.
M&MVI has some of the most brutal areas in the history of CRPGs, while M&MVII has one of the best minigames ever in Acromage, and you can pick up various ores to make better weapons and armour.

So to conclude I'd say M&MVII is more newbie friendly and is the best choice is you are only going to play one of the games.
But you really should play both.

I have only played 6 in the past and I loved it. Got all the M&M's on the sale and am now playing 2. You might buy the 6-pack first. M&M 4 and 5 are supposed to combine to make the world of Xeen which was pretty awsome in it's day per my brother who is a hard core rpg'er. (he's done all the Utlma's too etc).

I bought both around the same time when I originally got them, and VI is the objectively better of the two, considering it's about twice as long (60+ hours), and therefore allows for a longer, more epic adventure. However, VII is a more streamlined and polished game. It allows for more variety in party creation, it doesn't contain those hilarious/embarrassing digitized character portraits, and is generally easier.

You should probably start with VI, but if you have trouble you should maybe try out VII first. After that, VIII is probably the next logical step, followed by III or Xeen. I think VIII is alright, even if the fact that it was made in a year is painfully obvious throughout, and the difficulty is super imbalanced thanks to the presence of dragons as party members, who basically allow you to steamroll through the game no problem whatsoever. It's actually kind of sad.

VIII has some fantastic art work however... I would love to extract the art displayed while loading a game or an area and set some of them as background!!!!

M&MVI should definately be the choice since it also includes M&MI~V, those are also great great games.

Why not just buy all of them... They're all good.

VII has a better story and humour, a better character system, better graphics.
Go for the VI only if you prefer to just bashing monsters.

It's just my opinion, but I think that the character portraits in M&M6 are nicer looking than the bad CG portraits in 7 and 8. :^)
No offense, but I've gotta disagree with the idea that VI is only a "monster-basher." There's a good bit of story in there as well. Yes. Do this. :^D

Get both, then get 8 while you're at it! VI and VII are two of the greatest RPGs of all time in my opinion, totally worth the price

8 is rather poor compared to 6 and 7

Well, compared to 6 and 7 yes. But its a good game in its own right. I would say its worth the 6 bucks!

I prefer VI for the following reasons. In VII as I recall enemies were smarter about always targeting your healers first. I recall constantly having my cleric waste all their mana constantly healing themselves, and then still getting taken out, without any of my other characters even having a scratch on them. I mean, yay for the AI being smarter about targeting party members, but I'd just found it annoying at the time. The AI in VI seemed to be a bit more forgiving, attacking each character with roughly the same frequency.

The skill system in VII is also far more restrictive, which I found frustrating. In VI, if a character can learn a skill, they can learn it all the way up to master. A few of the teachers would require you to have finished a certain promotion quest first, but for the most part skills were very flexible. In VII not only were skills restricted by class as in VI, but how far characters could progress in skills were restricted as well.

In VI I recall the difficulty having a relatively uneven curve, which is good. Things would get hard, then you'd finally pick up the fireball spell and they'd get easier again. Then they'd get hard, and you'd finally get all your guys to mastery in bow, and they'd get easy again. It was a nice ebb and flow which gave you time to relax every now and again. VII I recall being all nail bitingly hard all the time. But perhaps I just wasn't patient enough with it, or I was too young at the time to come up with strategies more advanced than what VI had demanded of me.