like i sai, the Leo is cheap, lots of power, the OS is more expensive, lighter and slightly lesspower. power to weight you end up the same, the os will be more floaty tho.
APC make the 12.25x3.75 funfly prop, however neither the OS or Leo will turn it without a mini pipe, the APC 11x4 is also too much, for me a Leo ran best on a MASTER! 11x4 bags of grunt when poping out of the hang, put Irvine 39's to shame.

If i had to money, i might go for the OS tho.

cells, most places now sell lightweight Hydride cells at 600-800 mAh range without having to self build, i put a 1400 in mine, same weight as the Ni-cad, but you last much longer. Hydrides need to be watched more closely tho, no leaving them a week then go fly.