I wouldn't want you to think that there is too much merit in these comments from a mechanical engineering viewpoint, except Gubbs3's one about the weight distribution. Because mechanical gears are very efficient at transferring power, you'll not notice any difference that can be measured on the clock.
Power transmission gear designers (and we're talking gears that are used for power generation, pipeline pumping and seawater injection - 5 to 30MW) will go to great lengths to get their gearboxes from 98.4% to 98.6% efficiency, because that's worth a whole lot of fuel over a 15 year life! Usually though, it's the bearings that absorb the power more than the gears themselves.
The only benefit we get that can be felt is that the larger the pinion, the more tooth 'flank' is in contact, and therefore the less likely you are to strip a spur. So, given your choice, I'd go for the 125/37 as being the least likely to suffer damage in an accident, on a bumpy track, or, for Off-Road, landing from a jump. HTH
(As an aside, if you have a choice, the best involute tooth forms I've measured are on Kimbrough spurs and RW pinions. These will give you the best efficiency. HTH too!
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