I think sliding door takes up less room, I just put one in my laundry/bathroom. We brought the frame and sliders as a kit from masters and purchased door separately just their cheap internal door, as the door hangs from runners it is not quite touching the floor I don't think their will be any water issues

Either one serves it's purpose for different situations. I sometimes prefer stacking sliders because when they are closed (which is most of the time), they have wider glass panels, so form nicer windows. But bi-folds give you a wider opening when actually open - eg. 100% open rather than just 2/3 open. Stacking sliders can usually have sliding flyscreens as well which is sometimes a preference depending on which part of the country you're in if you want to keep insects & flies out!

Love that look but them do seem really expensive :) all the best !

I couldn't tick one or the other of the boxes – I think the choice has to be made contingent on so many variables. In fact we have painted timber, double-glazed bifolds (with a fabulous retractable pleated fly-screen) at the end of the dining room leading out onto a terrace and a painted timber, double-glazed slider (with a timber-framed sliding fly-screen) on the other side of the kitchen (but on the same axis) opening onto a raised timber deck. Each works really well in terms of function and aesthetics but they would not be interchangeable as the different locations determined which was more suitable – but they have been made by the same highly skilled window craftsman and have given us nothing but pleasure. And on a hot, steamy evening with both open we have the most wonderful cross ventilation.