I like Peter's plan. All the locations will become repetitive within a few hours due to the nature of the network in Glasgow (high frequency short routes). If you're only going up for the day, take the opportunity to visit all three stations and sample all that the Glasgow area has to offer!
GLC has lots of 156s, 314s, 380s and the odd 320 from Scotrail, as well as 185s, 220s/1s, 390s and a daily 91, not to mention the sleeper. Hyndland has a high frequency of 334s, 320s and 318s (and is probably better for photography than Partick). And GLQ high level has a few 156s, and plenty of 170s and 158s. A good mix I reckon, but there won't be any freight.
Might be worth heading over to
Scot-Rail.com and keep an eye for anything unusual that might be worth seeing. Usually plenty of gen on there (free membership is needed to read the posts).