You'll be able to spin larger and/or more aggressive props with the 1.25. This, in theory should give better performance. As Bruce88123 pointed out though, there is a weight penalty and that may or may not cancel out any gained performance. You'll also run into balance issues. Those are correctable of course.

the 1.00 will be great. I fly mine with an OS .91FS and it is plenty fast.

Take your time in working up to that Mustang. It is not a beginners plane by any stretch of the imagination. I've gotten a number of flights on mine now and I will say that it flys wonderfully once in the air. I haven't flown many planes that were as stable and smooth. Taking off and landing are another story however. It is a scale P-51 which means it is very squirelly on the ground until you have some speed and the rudder becomes effective. Landings generally need to be a little "hot" which means chopping the throttle to idle isn't always the best option. You also have to be sure to get the touch down just right or it can bounce like a basketball or nose over. That's the nature of warbirds. Not trying to scare you, it is all learnable. Just don't try it too soon and destroy that beautiful airframe.