The plus side to swapping to 5 lug hubs (So I'm told) is that you have a larger variety of wheels available to you, as well as a larger variety of brake kits, and having an extra lug to hold the wheel to your car is always a plus. If I didn't have ten million other things to do to my MK1, I'd probably look into doing it myself.
MK2 wheels aren't exactly cheap either (at least 93+).
More wheel choices for 5 lug? I don't know about that myself. Our MK1s enjoy enormous wheel selection, thanks to the fact we share similar sizes and offsets in the same bolt pattern as the Civic, Miata, and E30 BMW, to name a few.
I was close to doing this to my car. I thought using the 93 wheels would be a nice touch on my mk1.5, plus I could run upgraded 93 brakes and be done with that system. But there were some problems with the geometry of those mk2 hubs. I got nervous about the changes it would make, and gave up on the idea. Do a search on that mk1.5 that did this and you'll see that my concerns were valid. I'm sure he'll sort it out, but initially the handling was downright dangerous.
So I got 4 lug wheels, and am going a different way with the brakes. C'est la vive.