In the end (over a longer period of time) it will even out, whichever you choose.
If you're mining on a smaller pool: payouts will be bigger but occur less often
If you're mining on a bigger pool: payouts will be smaller but occur more often
...
This is the conventional wisdom, yes, and I don't necessarily disagree with it, but in my experience so far (admittedly not much), the longer the average time to find a block on a pool the lower the payout I receive, regardless of how long I mine at the pool. Theoretically there should be times when I get much more than the average payout balancing out the times I get much less on these smaller pools, but it seems that only the much lower payouts occur for me.
The most extreme example for me so far has been mining VTC on the give-me-coins pool which finds a block every 30+ hours or so; from a statistics standpoint I would need to mine there for nearly a year for my earnings to average out, and in the immortal words of Sweet Brown, "ain't nobody got time fo dat."
