Rider or Caster GSSR

Well, more than compare who you would *like* to get, it might be better to think about who you *wouldn't like* to get. By which I mean you'd have to consider how you feel about the others in that class. So, if you're torn between Riders and Casters, the Rider summons would have 1) Drake; 2) Ozy; 3) Medb; and 4) Quetz. Caster has 1) Tamamo; 2) Waver; and 3) Sanzang. If you really want Quetz, but don't want the others (either have them or don't care for they gameplay or look) you'd have a 75% chance of being disappointed. If you want Waver and Tamamo, with the Caster class you'd only have a 33% of being disappointed. (Just as an example, you'd have to look at your wants yourself to get the odds.)

So which has the highest likelihood of making you happy? For me, Nightingale is one of my most wanted servants. I was really upset last March when she was released and I didn't get her. Then she wasn't who I got in the summer GSSR. But despite that, I don't want Cu Alter or Vlad. Don't care for them at all. So a 66% of being sad means I'll be going Caster, where I have no 5* and everyone is either amazing value for gameplay (Waver and Tamamo) or a great character (Sanzang). So I won't be disappointed no matter who I get.

yeah i have the same problem.
i really like all servant in rider banner, even if end up whit medb i would be happy (or meaby she will, becouse she would be reunited to her cu alter)
but i know how strong waver and tamo are.
still i dont want sanzang....
but caster are so usefull...
but if i get my crazy francis....
yeah but 3 turn farm whit my waver
but ozy for my cleo...

That's entirely up to you. Personally I would tend to agree with the first post, whichever will give you the least disappointment. Both banners are very good. Gameplay-wise, almost everything in the rider banner is great, with the exception of Medb. Meanwhile in the caster banner, Waver is universally good, Tamamo is unparalleled at what she does, and Xuanzang is also (arguably) the best at her role as well. From a gameplay perspective, I'd say if you haven't got any of the servants on any of the banners, caster is slightly better, though you are right that Quetz is story-locked, and they always say to roll for who you like. Sorry if this just made the decision harder for you lol, but tbh it really is very tough to be in your position.

You do not have both tomamo and waver? The choice is obvious. go for caster.