Solo mining is technically always the highest payout on average. The problem is a matter of scale. At 1 GH/s on the Bitcoin network at this difficulty, it would take you about 8 months to find a block. You get the payment all at once, nothing before you find it. It could take you 4 years. It could take you 4 minutes. But "on average" it would be around 8 months.
The question is are you willing to make a gamble with that kind of payoff time, and are you really sure the market & network speed will be roughly stable during those 8 months? Because if you don't find a block early when network difficulty is rising, that 8 month average will keep increasing. If the market crashes before you find your block, you'll end up with less USD/EUR/etc. than you would have if you had been mining at a pool and selling the whole time.
I'm mining LTC and according to my pool I find a block every 24-36 hours.
I'm providing the response as it relates to Bitcoin (since this is the Bitcoin forum, not Alternate Currency forum). The same concepts apply for LTC, just on a much smaller scale since the network (and thus difficulty) is extremely small in comparison.