Solo mining is undoubtedly better mathematically, but you could mine for years without ever finding a block and I just can't see that sitting well with most people.  This is a bit of a simplification because I am not taking difficulty increases into account and because I am assuming every day as an individual trial instead of every share, but the average time to solve a block with a hash rate of 1ghash is 68.5 days at the current difficulty.  If you convert that to a percentage you have a roughly 1.46% chance of generating a block in any given day, or a 98.54% of not solving a block today.  

Chances of solving at least one block, at constant difficulty and 1ghash/sec:

1 day : 1.460%
10 days : 13.676%
50 days : 52.064%
100 days : 77.022%
200 days : 94.720% (1 chance in 19 of having a run this bad with no blocks)
365 days : 99.534% (1 in 214)
500 days : 99.936% (1 in 1,561)
1000 days : 99.99996% (1 in 2,436,681)