<font face="Verdana, Arial, Helvetica, sans-serif" size="2">So, my question--which would you suggest between PRG, BRU, ZRH or VIE as our mid-week destination? We're looking for quality of the city not just night life.</font>
Prague is so very lovely, but it's like a museum - not much happening, and you have to be careful - personally, I was there in 1992, and I got drugged having a glass of wine in a wine bar in a residential area("vinarnia" I believe is the Czech word). Everyone should go once, though, simply because it's so incredibly well-preserved and European. In planning your trip, be mindful of the recent flooding. Gay-wise, don't expect much.
Berlin is a BIG blast, you'll love it - that's on my short list of favorite cities anywhere - great museums, arts galore, great food - would love to go back.
Haven't been to the others, but have heard they're all a little tame. Assuming you want some place engaging and interesting AND gay-friendly, 1. do Brussels - food is probably best there, along with people with jobs and money to support an interesting scene; or if you have flexibility as to cities, 2. Hamburg or Budapest.