For me personally it needs to have both: shower over bath. I think for marketing it probably depends on how many bedrooms: if it has 2 then families might want to rent it and will probably be looking for a bath, if it has 1 then a shower would normally do.
For the rental market I think a shower room is more imperative - plus if you own the flat underneath its less of a risk of the bath upstairs being left to overflow.
Personally though I would like both - Shower is great for quickness in the mornings but a nice big bubble bath and a glass of wine and candles is the future!
Always think this argument is hilarious worrying. You can't get property clean in a shower as you need a good soak though bath followed by shower is obviously the best.