I'm running 16 GB Patriot Class 6, and quite happy.

A couple points:
The difference of SDHC vs. SD is that SDHC uses different addressing. So an 8 GB SDHC vs. a 16 GB SDHC will cause no problems -- if it crashes on SDHC, it crashes on SDHC, not the capacity.

I don't know, not having done side-by-side comparison, but I know of no reason to suspect that a higher-capacity card would eat more battery, etc. How would it?

Reasons for 16 vs. 8: It's not that much more money, and saves having to move all your stuff to another card later when you fill it up. Especially with music or video, you can burn 8 GB pretty fast. (And map images, too, if you use maemo-mapper much.)