Play the first. I like the first's combat more than the second, and I'll tell you why.
In ME2, you were just as accurate and your guns were just as strong at the end as they were at the beginning. You did not get more accurate, your guns didn't get stronger, they stayed the same from beginning to end. There was no sense of progression, no sense of getting stronger. Yeah you got a couple of new guns/powers to play around with, but they didn't really change up combat. Like, at all. You still stayed behind cover and shot dudes who poked their heads out(or casted your powers, depending on your class). The only two classes that were fun to play were Vanguard(who can charge everywhere) and the Infiltrator class(can turn invisible). And even then it was still pretty "meh".
In Mass Effect 1, you start out with ass guns, ass stats, ass armor, and pretty much just ass everything. But as you progress, you get better. Your weapons get better. Your skills get better. Your team gets better. You have an actual sense of progression. At the start I had to fire in bursts and couldn't cast much powers. By the end I could fire for 2 minutes without the gun overloading(not counting the "overload" power, which boosts accuracy/lowers heating up even more), my guns were super-accurate, I had such beefy armor I was like a tank. I went from "standard soldier" to "uber-badass". And it was great.
You also learn how the combat works. At the start you'll fumble around, but then you'll learn it. Learn when to use your powers, when take your shots, everything. You get better, Shepard gets better, you both get better at the same time and it just gives a sense of immersion that no other game has ever given me. Most people try to play Mass Effect 1 as a straight up shooter. Casting powers all at once, running in, ect. and I think thats why there was so much hate for it.
Mass Effect 1 is not a shooter. It is an RPG(although that in itself is debatable) with shooter elements. If you play it as a tactical RPG, pausing while playing, issuing orders, managing powers, ect. the game's combat gets great, fun, and interesting.
In ME2 you start out as "so-so badass" and just stay that way through the whole game. You don't get better, don't get more accurate, don't improve your guns(all the guns are basically side-grades instead of upgrades). There just isn't any sense of progression. Yeah you level up and get a bit more powers, but they all have the same cooldown for some stupid reason, so you'll cast one, then wait for everything to recharge, and then do it again. Its boring.
Again, ME1 is not a shooter. If you don't like that, then guess what? The game is not for you. This is not a bad thing, it just means this game is not for you.
ME2 is a shooter. I guess it would be an action adventure, since it has essentially no role-playing. Same with Mass Effect 1, only ME1 is just adventure with shooter elements. Again, ME2 being more shooter-like isn't bad per-say, I just found it rather boring.
Just my two cents on the matter.
Also ME1 had infinite ammo. And no matter how you say it, I don't see how going from unlimited ammo to limited ammo is an upgrade. Especially if you like sniping, in which case you'll be running out of ammo very quickly and have to just go up the front lines and fight Gears of War style anyway.
And another thing, I seem to be one of the very few people who think naturally moving in/out of cover is better then pressing A to stick yourself to a wall. In ME1 if you want to take cover you go up to a wall, and Shepard automatically gets into cover. In ME2 you have to tell him to do it. I guess TIM didn't fully repair his brain if he doesn't have the sense to get into cover when getting shot. But thats just a small nitpick-rant, so take it as you wish.
