i think my votes for the 61FX over the TH75. i have the 75 and it's given me troubles since day one. i wouldn't wish it (my particular one) on even my enemies. i finally decided to retire it and replace it with an O.S. Surpass 91, which is a great engine.

if you already have both, and the 75 runs reliably, then i'd go with the 75 for the power. if you haven't bought either, then i'd spend my money on a fourstroke engine like the:

(listed from most economical to most amazing)
magnum FS 91
thundertiger FS 91
o.s. surpass 91
saito 91
ys 110 (that's the only one in the area)

the surpass and saito are about equals in price but saito's love high nitro fuel so you're looking at higher running costs with higher performance. the surpass is happy sipping 5~10%

personally, i'd go for the magnum FS 91 since it offers decent power, good reliability and great 4-stroke sound that, at a price of 199.99 at tower, is only $30 more than the 61FX

if you must go 2-stroker, i heard good things about the supertiger g-75 ringed engine.