It depends on the type of diving you do, and if you have a 'hampster bladder'.
The longer your dives the better a 'Pee valve' is. By long we are talking about 3 hours underwater - long deco obligations. If you have to hang about decompressing for 45 mins then you have nothing to do, and once you have started thinking you need to pee..... there is no stopping you! The disadvantage of 'pee valves' is that they aren't overly reliable and rely on glueing a urinary sheeth over you!
Relief zippers on the other hand can only be used above water. For me this is the better solution - you can spend all day in your drysuit, relieving yourself as and when, but it can't be used in the water - so, if you can 'hold it in' (!?!??!!!) this is a good solution.
Diapers / nappies work well as well - they are really an alternative to the 'Pee valve' and can be used underwater. However, once wet they cool down and get uncomfortable. Also not really good for wearing all day.
So, decide on what you want by
Long underwater - nappies/diapers or 'Pee valve'
Just plain need to use the loo whilst on the boat - relief zipper
Of the nappies/diapers vs 'Pee valve' I would suggest that provided you are relatively discreet then the nappies/diapers are probably better. However, you still have to go and buy them!
Jon T
(PS I will get a zipper fitted when I replace my drysuit)