Yeah I know but with theatre, sometimes you have the main cast and a backup cast, so say Wednesday lunch you're likely to not get the "real" lead performer but a stand-in instead, and some times and days the crowd is smaller which could be both a good or a bad thing...
Saturday night's the most expensive one AFAIK and they actors probably get more 'geared up' to the Saturday performance so it might be a better performance overall, but sat night you're gonna get stuck in huge London crowds...
That's what I meant by best day/time, I know it's the same show whatever time you see it, but there are variables and figured someone might have an opinion on the best time to see it