Hi, Richard,
I would clearly say that epoxy grout is the most durable by a pretty good margin. That said, it is pretty difficult to work with and it may be troublesome finding someone to use it.
Normal grout (I used Custom Building Products from Home Depot) has held up flawlessly for me over a seven year period so I am reluctant to criticize it (or any other latex fortified grout).
From the description of your water source, it's beginning to sound like the pool water might be "starving" for Calcium. When that's the case, the pool water will literally leach the calcium from the grout to satisfy it's demand.
I'm pretty sure epoxy grout cannot be leached regardless so, if you can get it put in, that may be the best bet but I think if you can keep your Calcium Hardness up around 250-400 and your pH around 7.5 or so, you will likely eliminate the problem.