What ever way you go, I'd get away from nimh as some point. My big problem with the nimh is two fold. First they are heavy. Second, then start to degrade very quickly, relatively speaking. I've seen nimh cells start to degrade after 10 cycles. The lipos and li-ions are more cost effective in the long run and they perform better to boot.

The A123 cells are nice. They'll take a beating and charge quickly. The downside is that they are large and heavy with limited capacity. My 5S2P 8Ah pack isn't much heavier than a 5S2P A123 pack and has almost twice the capacity and lots more power. The 5S A123 pack is a closer comparison to the 4S2P 8Ah pack in both weight and power delivery, but you get more runtime with the lipo pack.

As was said above, you can build yourself a 5S2P A123 pack for about $140, if you consider the balancing tap, cells, wire, etc. Although I'm very interested in the A123 cells and will likely get some in the near future, the ease of just buying a 4S2P 8Ah lipo packs keeps me coming back for more.