Let's say that a user has a decent amount of computing power. At the current difficulty of 1777774.4820015, he could have for example 10Gh/s (he would earn around 5 BTC per day according to a calculator).
Is it more profitable for him to solo mine or to pool mine? What are the differences in this situation?
It completely depends on the "pool fee". Theoretically, with a zero percent pool fee, solo mining and pooled mining should, over the long term, produce precisely the same revenue. The only exception is that some (most) pools keep the transaction fees for themselves.
If you mine solo, with an expected 5 BTC/day take, that will mean on average you'll mine a 50 BTC block every ten days. It will be completely random though. You could mine two blocks in a day. You could go three weeks without a block. When the difficulty changes, not only will the amount you get paid change (that always happens) but the time between payments will change drastically as well.
If you mine in a pool, you take the pool fee right off the top. A 3% pool fee means you make 3% less. Also, most pools don't pay transaction fees. But your revenue is more predictable. You'll get paid on a regular basis and your payments won't vary much (until the difficulty changes, of course).
One advantage to solo mining is that it's more reliable. Pools have outages and have had a problem with denial of service attacks lately. Mining solo, you aren't relying on other people's systems to keep your mining going. If you pick a very reliable pool, or use a mining proxy with a "fallback pool" configuration, this isn't a major issue.
One advantage to pooled mining is that you get to use the pool's tools. These include things like web-based control panels, alerts, and so on.
The rule of thumb I recommend is to solo mine when you have enough hash power to generate at least one block per day on average and use a pool if you have less than that. You can expect the pool to take between 1 and 3% for their service, but that's a better alternative to the prospect of being unlucky and not getting a block before the next difficulty retarget.
Solo is more profitable in the long run for the reasons David stated. And if you do the math, you will see it is independent of hashrate. Only variance is higher. But you should consider pool hopping as well for maximum profit. By always mining at the pool with the currently highest projected share price for the next block, you will earn even more than you would mining solo.
If the difficulty doesn't increase before mining a block, then solo mining is more profitable, because the pools normally take fees. otherwise pool mining is more profitable.
