total copy and paste job from my other post:
Mathematically and statistically, if you mine for a pool, you lose money 3 ways compared to solo.
First is the fee they charge and there are some no-fee pools. If the fee is 2% then for every 50 coin block you would have found solo, you'd get 49 BTC from a pool. BTCGuild has no fee options though so it's avoidable.
Second, you also lose the stale share amount caused by the delay from a pool server handing out processing assignments to people. Whenever one person solves a block, whatever everyone else is concurrently working on is invalid and they don't get paid for it. This effect is extremely variable based on your speed but in the neighborhood of 0.5% for most people if I remember correctly. So your 50 to 49 BTC block from earlier is now about 48.75.
Thirdly, almost no pools give transaction fees to their members (which better change within a year!!!!) so you'd also get like 50 and change for finding a block solo. It's not much really but you never know. I don't think there's been a 51 BTC block ever. The most recent block with a high volume of transactions was 50 + 0.064 total transaction fees for 151 transactions. So solo you'd have gotten 50.064 for solving that block. So maybe your 48.75 pool mining block is now a 48.74 block on average. If you have a 21GH/s system, you're potentially losing 1.26 BTC every 4 days if you're mining in a pool. If you have 1 GH/s solo mining, you're missing out on 1.26 BTC every 3 months so it's not a huge deal. That's loosely based on a 12,000 GH/s total, which I'm not sure is real accurate.
BUT, solo mining and failing to find a block then having the probability decrease due to an increase in the entire rest of the system's total GH/s has diiiiiiire consequences. It's more detrimental of a result than the math appears to be on the surface but I won't bore you with the mini-research paper on why
It's sort of a like a inverse of the old gambling "but I can stop when I win so that screws up the probability calculation" thing. I'm sure there's some term for it.