I suggest the following:
1) Look at each consoles library then look at current release schedule for this year - determine which appeals to you more vs what you play on Wii (one caveat on this is do you have a decent gaming PC? If so then consider whether you would prefer to play games like Bioshock / Mass Effect on PC or 360, if PC then drop them from 360 games list when comparing)
2) Consider online vs offline - are you planning on playing online? Do your friends already have console for online? If so and you want to go with friends follow their choice so long as it doesn't affect point 1) above. If you want to go online and don't care about friends then its cost vs promise. Online with PS3 is free but 360 has better online but you pay for it
3) Consider non-gaming elements - is this console only really for games or are you thinking of watching movies, storing content, etc? Are you interested in BR? If so then PS3 might be better, if not then 360 is probably more in line with what you want from the console
4) Consider reliability/risk - the 360 should I believe still be considered as a higher risk from rate of failure point of view. Does this matter? Do you want more solid console or are you happy with getting console replaced? If you don't like risk then PS3 is better choice than 360 for reliability
If the above fails toss a coin and don't blame me whatever happens...
For the record I wanted solid console, non PC centric games potential, DVD/BR movie playback and free online so I got a PS3. I also have a PC for games and removing the games such as Gears, Bioshock and Mass Effect from the equation I realised most 360 games that were exclusive that I wanted to play I could do so on PC while games like Uncharted and R&C would not come to PC so in fact the smaller PS3 library was actually larger from my point of view than 360's.