There are a few different spoilers out there... I've got a thinner one which does not have a 3rd brake light (nor the space where one should be), and then there is the more commonly seen thicker M3-style one (usually with 3rd brake light in the centre). I don't know what type mine is, don't think it's M-tec as that['s the M3 spoiler on non-M e36s'... I think? But I have seen it on a few BMs in my area.
The boot width is the same size so either should fit saloon and coupe, however if you're changing from one type to another then the screw holes might be in the wrong place. It would probably be best to ask for measurements of the spoiler you're looking at buying if you're replacing an existing one, just to check it will still fit.