As for the speed difference, it's not as much as you'd think.
A respected US bow designer who I've been lucky enough to meet and correspond with told me about the tests he ran to investigate this subject. With all other factors being equal (draw weight, draw length, arrow weight, etc.) he tested 4 different bow lengths for speed - 64, 66, 68 and 70in. The limbs were identical draw weights, but different lengths.
The difference in speed between each set up was 1 foot per second (i.e. the 68in set up was 1fps faster than the 70in set up).
Now, if I was hovering around 30in or just under, that would be enough to decide me on the longer limbs for the extra smoothness. However, I'm hovering around just under 32in at the moment, so my next set of limbs will probably be 71in
