For me, while I prefer XT just for usefullness, were talking about building up old bikes. If its just a hack, i'm on a budget build then neither. But if its going to be something nice or that you like, then the whole XTR is only for racers talk is silly, if you're thinking that then a new bike with SLX on it will do an even better job. So if you are debating XTR vs XT, then XTR all the way (assuming it is in the budget).
Not that I will take my own advice of course.