ME 1 has more RPG elements. longer missions (better side missions), more items more exploration and the conversations seem to have more effect. You can also gain more levels and there are more options in character skills and powers - it is not unending like the elderscrolls, so at some point you can't just level and level and level everything end seventually and you're only option is to complete the game.. However, there are fewer characters. ME 1 does have some problems - there is a permanent and apparently unpatchable graphics glitch with one character's face that essentially makes it low detail no matter what. exploration especially controlling the tank aren't great and (surprise) it's older, so it's not as pretty.
ME 2 is more a shooter than an RPG, there are few guns, but the differences are more noticeable. head shots matter, but there are fewer powers and they aren't nearly as useful -though they are easier to use. one downside is not everyone can use every weapon. if you want the best rifle in the game you have to pick the right class. The conversation and story telling seems more forced, but there is a bit more variety in what your character can do or say (it feels like it matters less....) Another thing to consider is exploration, while ME 2 does have it, it's done by spaceship, and it costs money, and mostly it only gets you resources. But without loot money is finite (essentially) so every time you explore you worry that you might not be able to afford a ship upgrade that might save the life a teammate, a dynamic that stifles one of the most important parts of being in space. This, in turn, pushes you toward the DLC (of which ME 1 has only 2 -the first good, and free, the second bad- me 2 has.... well, alot. they're pretty good, but it feels you're being nickeled and dime to get to play the whole game and all of it will cost about an extra $50, give or take)
For me, however, the biggest issue is story telling. ME 1, I feel, does a better job at telling a compelling story and instilling a sense of awe. It lets you know how big space is, it makes you fear the enemy and wonder at the marvels of aliens. ME 2 is pretty with more complexity, but it does a poorer job at making you care about characters, threats or the moral decisions you have to make (though overlord DLC is a awesome excpetion, so if you get ME-2 get that one. definitely).
In short, if you like the RPG elements, or if you want to invest in a good story get ME 1 (at least get it first). If you just want a space shooter with pretty graphics, get ME 2.
hoped that helped.
PS cheating in ME 1 is easier, in ME 2 it's neigh impossible. Also, you can't redistribute points for your character in ME 1 so if you want the same guy and the same class but different skill set you have to start all over. In ME 2 you can buy a skill reset in game (or even buy a new special skill)