for looks, it is going to be SLR; I have pair, later model with the machined sidewalls and with after market green Ti skewer (skewers should not be part of you decision since there is a vast choice). you will need special spoke key for them plus the bearings are a known weak spot and not easy to replace (need to be pressed in).
for performance, XTR is a good as it gets in my opinon but not exciting to look at - the SLRs are not as light as you would expect/hope - are you able to weigh both pairs? (SLRs are in the regoin of 1450 g). anyting under 1400g is good going.
SLR have more potential to shave weight by means of Ti rotor botls and lighter rotors e.g. hope floating or kcnc razor (anything less you are talking aluminium alloy/Ti disc and many ££££s) ; in comparison XTR centre lock are a fair bit heavier.
The rim on the SLR should be undriled so are tubless ready.
the sensible solution is to fit the SLRs and buy another frame for the XTRs