So we're some of the first to get our hands on Mass Effect for the 360 and Uncharted for the PS3. Mass Effect is a TPS/RPG hybrid and Uncharted is an AA game. If I had to think of a simple way to compare them to other games it'd be something like: Mass Effect is like Knights of The Old Republic has a lovechild with Star Trek and Uncharted is like Tomb Raider and Indiana Jones got into a blender and made a videogame smoothie. Both with very unique outcomes.
I know a lot of people have short attention spans so I'll sum up everything I'm about to say right now: Buy Mass Effect, definitely at least rent Uncharted.
While Mass Effect is not my usual cup of tea, it a wonderful game that combines superb graphics with an innovative combat system, great voice acting, fun gameplay, a convincing musical score, a well told and well thought out plot, and best of all it stays true to Bioware's style of letting you choose whether to be a good guy, neutral, or a total dickhead. My only real complaint is that I'd rather not cross genres when it comes to my RPGs.
Uncharted has got some of the best environmental effects I have seen ever. If you jump into a lake your clothing becomes visibly wet. If you pull out your pistol and shoot at a toucan it will squawk and fly off in a panic. The gameplay is fun, which is all you can really ask for. The game's NPC intelligence is top notch. Enemies will duck and cover in reaction to your offenses making it a real challenge to take them out. The voice acting is good as are the ambient effects around you. My only real complaint is that the game is much more muscle than mind. As fun as slaughtering can be, sometimes a challenging puzzle is equally fulfilling.
Much to my dismay both games suffer from "next gen" syndrome -- being too short. But! Don't let that deter you. Uncharted has a lot of replay ability through unlockables and though the main story of Mass Effect can be completed in around 17 hours that does not include side quests, hidden content, or varying scenarios, all of which you will definitely want to uncover.