My glass bricks are in a wet area... mortar or silicone in this case?

Let’s list the fors and againsts…
Mortar Systems:
For:
Very solid wall, excellent for free standing situations where the end of the panel is not fixed to a wall.
If you want to match grout colour with surrounding tiles, there is space to apply a tile grout over the top of mortar joints.
Against:
Unlike tile grout, there doesn’t appear to be mould inhibiting products to add to a mortar mix, which means that you either have to clean very regularly and occasionally use products like Exit Mould to remove it, or apply a tile grout over the top of the joints to keep it at bay. (There doesn’t seem to be any way to eliminate having to clean mould of bathroom surfaces but you can slow it down a bit)

Silicone Systems:
For:
Small joint sizes (3mm for silicone vs 10mm for mortar) means you can match a theme with tile joints being a similar size.
Good mould-resisting silicones available (not mould-proof, you still have to clean it from time to time!)
If mould does affect your silicone joints and you can’t remove it, you can cut the joints out with a sharp knife and re-apply new silicone.
Against:
Free standing shower screens are not as rigid due to the flexibility of silicone.
You can’t use tile grout for the joints to match tiled areas as it will eventually crack due to the flexibility of silicone systems.

Planning to install into existing openings?
The fors and against listed above are less of a concern than fitting into your opening without having to modify it, which is obviously adding expense.
If you contact us, with some (accurate) measurements (approximate isn’t helpful unfortunately), we are more than happy to help you decide what will work best for your project.

You can always get us to come to you to provide a free onsite quote.