Hi there
As for spotting, generally there would be more traction to see at Edinburghs Waverley Station than Glasgow Central, due to more open platforms. It really depends what you are looking for. If you want to collect units then Glasgow would be good as Central and Partick stations would allow you to do this very quickly! If you are looking to see anything in particular (freight, HST's etc...) then it entirely depends on what you want to see.
If you are looking for WCML traffic, I would recommend heading south (if possible), to somewhere like Crewe, as this is a very busy station serving most of the core WCML routes (Scotland, Wales, Liverpool and Manchester)
If you want to go more for shopping, then I'd recommend Glasgow, as there are large shopping centres, as well as Buchanan Street and the Merchant City.
If you can give us an idea of what you are looking for, we would be able to point out specific places to visit