Just ask yourself if the E38 is worth the extra £750 schmoolies. £750 buys a whole lotta tyres, insurance, servicing etc.
A '94 E38 typically sells for getting on for double a last of the line '94 E32 so if this one is only £750 more then be suspicious.
I wasn't prepared to pay more for the same age car so I went for the E32 which i think actually looks better - less of a Euro blob.
Also, what engine configuration do they each have? They could each be either straight 6 or V8. The straight 6 is the tougher unit and easier to work on but less powerful. Like for like, the E38 is no faster than a similarly engined E32.
Compare interiors thoroughly on each car. Some find the E38 a bit plasticky, especially the nasty plastic panels attached to the drivers seat to accomodate the controls. Others find the E32 a bit old fashioned, although I think
the '32 was slightly more spacious.
I think it comes down to economics and I can't see why a car the same age should be worth more purely because it is a later (and not necessarily better) model.