5S2P will give you more punch, but it's kind of an unfair comparison.

In any case, if 10 cells is your limit, then 5S2P is the way to go. If you are considering going with the A123 cells, you really need to decide if they are indeed what you need, or whether they will acutally give you any benefit.

As you noted, they are very heavy. A 5S 5Ah lipo pack will cost you about the same as a 5S2P 4.6Ah A123 pack, but the lipo pack will give you better runtime and a higher voltage.

The lipo pack also weighs juat a bit over a pound and the 5S2P A123 pack will weigh closer to two pounds.

The A123s offer a quicker charge time, but only if you have a charger capable of allowing you to avail yourself of that benefit. I run mostly 5S2P 8Ah packs and get around 45 minutes of runtime with those and they charge in an hour. With the A123 pack, I'd get around 25 minutes, but they charge in around half an hour. Until I can get a charger that can charge at a 20A rate, I'm going to stick to lipo.