I think using sapphire glass doors on tub shower units are functional, make the space look more modern, and also make the bathroom appear larger. Plus, if you're doing any contrasting tile layouts between the field tile and the shower niche, it will show off your beautiful accents. Here's an installation recently done for a client doing just that:

Great point, Mark. I think we’ll skip the glass.

I prefer a curtain. Easier to clean and with the right curtain, can look very classy indeed. Some of this will depend on the actual style of your bath, so if you'd like to drop in a photo of your actual bathtub, that would be cool.

And as Mark pointed out, if one does need to bathe a toddler in one, a curtain is much more practical, so it will depend on if you foresee the need, or if you plan to sell in the near future, or if there's another appropriate toddler bathing spot in the house.

The other thing I will note -- in one of the homes my parents had, the guest bath had a glass door. The crevices were a nuisance to clean. In fact, though my mother was a neatnik, there was always a tiny bit of bleah remaining...

Yes! Glass doors. Esp if its not used much - glass is hard to keep clean sometimes due to soap scum. Dreamline has some really cool looking frameless glass doors which actually has a coating to resist soap scum. I particularly like the look of the partial glass doors or the type with the hinge swing out doors.