Price?
K10D isn't too much expensive than K100D, and I think it's a great bargain for its price (especially when it was initially released in SG last Dec).
Size and Weight?
When mounted with a big lens, the weight difference can be almost ignored imo, actually the bigger size of the K10D can keep the balance better when a big lens is mounted. But if you prefer using the kit lens, or those cute DA LE lenses, the K100D definitely feels smaller and lighter.
Battery?
Well, after using D, Ds, K100D and K10D, I'd chose the dedicated non-AA Lion battery anytime. It's just so much reliable than the rechargeble AAs, and it lasts longer too.
6mp vs 10mp?
I do prefer cleaner high ISO photos by the K100D, but most time I use low ISO, so I guess I can live with that. Besides, Neatimage can clean up the noise quite easily when needed.
Here is the definitely winning part about the K10D:
weather sealed, faster AF, much effective SR, better built, easier to use, alot more features (and some of them are very useful), SDM support, and better yet, it got a Grand Prix version
Go and get the K10D if money isn't an issue, that's my suggestion. Or better yet, get the Grand Prix Limited Edition:
Cheers!
Here you go