To be absolutely sure you'd need to know the distribution of the weight, not just the total weight since I'm pretty sure it's not going to be a constant density across it. But let's assume it is for both(They'll have similar weight distributions I think, at least) IIRC the momentum in this case would be Momentum=1/2radius*weight
So, ignoring units just to get a relative number(I can't be bothered converting into the right units to get newton meter seconds)...The 17 inch will be 81.6 whatevers, the 18 will be 78.3 and the 65.45.
Basically, don't be a ricer, stay with what you have and put the money into something that will have more than a tiny effect.