Faster is better however it really depends on what plane, what surface, etc... .17 is not a slow servo and unless your flying extreme 3D then your not going to be able to tell a difference. I wouldnt go with less expo to make up for the speed. IMHO unless you fly like quique then your not going to tell a difference between a .14 servo and a .17 servo. Go with the digital servos you will thank yourself later and run it on 6 volts it will speed them up and make them stronger besides digital servos can be voltage hogs so if you go with the digis then I suggest a 7.2V Li-ion pack regulated down to 6v and run pretty heavy gauge wires. You can get Li-ions for a reasonable price here

www.troybuiltmodels.com

I use them and I think for the money they are the best.