Older Venoms are kinda bad, but I hear now their LiPos are pretty good. I don't know about their large-capacity NiMH packs, but it looks like most of the RC shops I've seen stock them. Could be they also offer the most profit margin for shops (lower wholesale $?)
I'm real happy with Elite 4500's from cheapbatterypacks.com - they now sell a 5000 and 4700 Elite cell and will custom-build packs for you. Yes, they are unmatched and I do find that one of my packs works better than the other.
My son and I also tried out the Tenergy Propel 4500 packs that were prebuilt stick packs. Not too bad for similar performance to the Elite's. Great for practice packs.
I really really want to get into A123. The "safe" lipo. I suspect that they may some day make a new cell-size (hopefully Sub-C) but most likely they are factory-tooled for one cell size. The 6.6 v 2S2P packs would be fine if you ran lower-turn motors (say in the Mod classes). Since 6-cell NiMH run about 7.2v when under load, then it's not too much difference - but the A123 discharge much higher rates. If your track will allow it, use a 3S or if it can fit, 3S2P A123 setup. I doubt it would fit easily in a 1/10 scale, but I've seen some videos of 3S1P A123 running in a T4 and it hauls quite nicely.
A local track near Philly had a Lipo fire in the pits on Sunday and apparently the guy lost the truck it was in and when it was burning, it fell near a 1-gallon jug of Nitro. So, LiPo must be maintained carefully. I've also seen a couple NiMH cells blow - not as much "excitement" as a LiPo fire, but still a bit scary.