I think that when you get up to a few thousand mounted slides, as I have, you'll be finding the Gepe glass mounts a bit on the heavy side too, and it affects the number of sleeves you can fit in a ring album or filing drawer so long term they increase the storage space needed quite markedly. And they're a pain since you invariably have to clean them before inserting the film, and they're very difficult to write on so captioning etc isn't easy.
That said, the glass mounts are the highest quality solution, since they keep the slide in position, flat and dust free, and they focus more easily and "pop" less in a projector. I've never managed to use the glassless 6x6 mounts in a way that stops the transparency slipping around inside them badly. The 645 version works just fine, but they have a much more substantiall metal mask round the frame than do the 6x6. What I actually use are self adhesive white card mounts. They aren't everyone's favourite solution, but they are light, easy to write on , cheap (under £10/00), very quick to mount and they project well enough in the Hasselblad and Cabin projectors anyway. Many of them aren't thick/stiff enough to project decently but those from Process Supplies (
Process Supplies (London) Ltd provide a specialised supply service for professional photographers) are rigid enough though I wouldn't like them to be any thinner. If you scan or print you end up destroying a mount- but they're fast and cheap to replace. Some say that they leave a residue of adhesive round the edge of the slide, but I have to say that this hasn't been my experience and no labs have ever commented to me about the amount of cleaning they have to do and I've never had a problem with a print of a scan.
I hope the picture I'm painting here is that all the solutions I know of are a compromise and you just have to decide what set of attributes, strengths and weaknesses suit you. Trying a box of each is not bad advice. I know a guy who puts everything he intends to print in glass, and those he only ever intends to project in card- but then he shoots 645 and the card mounts are much more rigid than the 6x6 version.