Eh... depends.
The PS3 has several great exclusives but, to me, the few 360 exclusives are miles ahead in quality. InFamous is really good, and Resistance 3 is damn fun, and God of War is pretty fun (all of them), and those are all system sellers. However, one "system seller" isn't enough for me and neither is 3.
Do I think that buying one is a good idea? If you've got the extra money, yes. If you've spent the last six months saving up money for a PS3 and games, then I suggest putting it elsewhere because you worked too hard for that to get a console that isn't going to give you anything worth that time you missed.
And, as a general rule of thumb, multiplats almost always run slightly better on the 360.
Note that I'm not saying the PS3 is bad. It's the furthest thing from, and having both is certainly advantageous, but the worth of having both is directly related to how much extra money and time you have.
EDIT: However
If you do not and/or have never owned a Playstation 2, then get one of those and a good number of those great games that everyone talks about ASAP. The PS2 sold so many systems for a very good reason, and thus far its collection has not been outmatched by anything that isn't a PC.
EDIT 2: And, despite the hype built up around it, Uncharted is hardly anything better than "average, but fun" in everything except its dialogue. Its dialogue between characters - not story, just dialogue - is pretty well done and the voice acting is excellent. But what's stopping you from watching all of that on Youtube?
That comes from the one of the biggest fans of early Crash Bandicoot and the first four Jak and Daxter games, so that comment should not be taken lightly.