The 750il is rare because it has a V-12 if not mistaken. Not too many cars were built... knowing this, maintenenance & parts are not cheap. You didn't mention the miles on either car....
The 530i can be a daily driver, while the 750i is more of a weekend/long trip type of car, pure luxury. It comes down to the type of driving you do and if you can afford the maintenance on the 750il as a daily driver. From a collector point of view, the 750il wins hand down over the 530i.
Rule of thumb for v-12 engines is to keep 2K for maintenenance issues per year as the car gets older and miles add up.
I almost bought a '95 at one point... It was nice!