Sounds like you really are going overboard with the 'Blad gear!
There really is no absolute "best" choice between these 2 - it depends to a degree on the circumstances and type of shot but mostly on personal preference and shooting style.
For me one of the delights of MF is viewing the image directly on the ground glass through the WLF. The laterally reversed image almost forces one to think more carefully about composition. This is, of course not always possible.
Eye level shots particularly tripod mounted require a level of gymnasic contortions which I now find unappealing and the traditional solution of standing on the massive flight case in which you schlep the kit around is fine in theory but falls down when you want to take the camera on a 6 mile walk. (This was a huge bone of contention with the first Mrs. Nicos, particularly when I not unreasonably asked her to act as sherpa although I am not convinced that it was a contributory factor to the eventual divoice).
If you find yourself wanting to shoot at normal eye level or higher then of course the 90 degree prism is the obvious choice.
The downside (and there is always a downside isn't there?) of the 90 degree prism is that for me at least the handling of the camera is somehow faster, easier and more intuitive when it is held a bit lower - I am certainly not alone in this having spoken to other MF users.
This is where the 45 degree prism comes in - for action shots where handling speed is paramount it avoids the possible confusion of the laterally reversed image while preserving a really comfortable stance and grip of the camera.
As you see it is very much "horses for courses" and the decision may be further complicated by the presence or absence of the winder attachment which completely alters the feel and handling (far more than a 35mm motordrive or digital power grip).
Ultimately the choice will be yours and I would certainly suggest that you buy neither until you have run a lot of film through your system to get used to how it feels in your hands and how well you get on with the WLF since if you bolt a prism on straight away you will not only be depriving yourself of one of the joys of using a MF SLR but I am of the opinion that it will make it harder to get the instinctive familiarity with the controls that you need to get the best out of your kit.
Having said all that, if you have your heart set on getting one straight away I would go for the PM90 as there are many more situations where it is likely to be useful.