RaZoR asked about Mass Effect, so I'm going to try to answer my thoughts here in this post (since it's tough to do a full 8 person party discussion while playing BC2).
BOTH are WELL worth the money IF you enjoy: RPG games and/or Science Fiction.
In both games you really feel like you are part of a massive story and you are helping it to unfold. Both games give you the ability to make choices that may come back to haunt/reward you in the 3rd installment. Both games introduce you to new aliens, and what I like about it is that you aren't considered the galactic good guys (Babylon 5 approach instead of Star Trek). In fact, many of the races look down on you and you will have to earn their trust (or prove them right) by your actions. All the races have big differences that are well-flushed out with backstories that make sense for their appearance/behaviors. It's why I have played through the game so many times - I would say there is just about as much depth in this game as there is in BC2. In both games you can create your own Shepard - his/her face, class (determines weapons/abilities), and background.
ME1 is more RPG-like, being able to switch out weapons and armor, and upgrade those with mods. Therefore, you are able to highly customize your character. Gunplay is secondary but it is TOUGH and rewarding. If you get good at chaining abilities with your squadmates, you can really roll through enemies and it is very rewarding. The MAKO, your vehicle, can be very annoying to drive. Almost all sidemissions are the same "drop on a planet, kill some dudes, fly away" which does get repeditive. The story is great, and you really don't expect what happens.
ME2 is more focused on shooter mechanics than RPG. Less customization options, fewer choices for improving your character when he/she levels up. At the same time, it feels much more streamlined with less time "wasted" in an elevator. Though suddenly you're back to a limited supply of ammo which does get annoying at times. You can now only use one power at a time, which if you enjoyed that aspect of ME1 you will dislike. The story is great, though, and you have much more control over the game. You have a larger group of people you work with, and decisions from ME1 will show through in ME2. You can even die at the end of the game (one of mine did LOL). The ending really makes you want to play ME3.
DLC wise, ME1 platinum comes with Bringing Down the Sky, which is pretty fun. Their 2nd DLC, Pinnacle Station, is fun, too, though lots of people don't like it as much. I almost like it more TBH since you get some great goodies from it.
DLC for ME2 is more diversified, with Lair of the Shadow Broker being top notch. Like BC2's "VIP", ME2 comes with the Cerberus pack free in new copies. It's nice but not 100% necessary. I bought Kasumi's DLC by accident for full price but still like it and am glad I did. Overlord is fun. Arrival is pretty good but a lot of people don't like it for whatever reason.
I've played both games about 5-6 times now. So far, my Shepards have been:
(Myself) - Male, soldier, choices based on how I would play.
Jean-Luc Shepard - Male, bald (duh), 100% paragon (good guy approach), Sentinel.
Jean-Luc Shepard playthrough 2 - Started a second one to get a boost from the beginning.
Megab*tch Shepard - Female, red hair (matches my wife's LOL), 100% renegade, Infiltrator
Megab*tch Shepard playthrough 2 - same as Jean-Luc Shepard 2.
Middle Shepard (current play) - Female, doesn't want to make a lot of choices, therefore picks the middle of the road options.