It appears that the 125 and 150 have the same max torque and there is only 250rpm difference in revs between them. The 175 maxes out at 7000rpm, and 6900rpm for the 150 is in the same area.
So if you have the money go 175, if you need to save a little go 150. You may well be dissapointed in the long run with 125.
You need as many horses as you can afford, you do not have to use them all the time but they are there when you need them.
S7MAD