| Don't pay extra for the Futaba brand. The others will work just fine.
Actually, I believe that the Tower Hobbies brand equipment is just rebadged Futaba stuff -- as far as I know, it's identical.
As for a 500 mAh battery pack vs. a 600 mAh battery pack, the larger one will get you a little bit more flying time between charges -- perhaps 20% more. If you're worried about it, buy or make (they're not hard or expensive to make) a new pack with better batteries.
Do NOT buy your battery packs from Tower. Buy them from radicalrc.com, batteriesamera.com or your local Batteries Plus. Or make them yourself (if you know how.) You'll save a lot of money.
| While you're at it though, you should go on ebay and look for 1100 | mA packs for the same or less.
Or you could get one made with 1800-2300 mAh NiMH cells for a similar price. The downside is that it'll take a looong time to charge if you don't have a fast charger. Of course, you won't normally be charging from nothing, but still ...