Rider or Archer for GSSR

Only a 33% chance of a st archer in Orion as tesla and arunja are AoE. And only a 25% chance of an AoE rider with drake as ozy, medb, and quetzl are st.

Didn't you get Santa Salter? She is an excellent AoE rider.

... The "which servant" part of the question implies that you might think we have a choice. We don't. You get a random 5* non-limited member of the class: Orion (ST), Tesla (AOE) or Arjuna (AOE) for Archers; Drake (AOE), Ozymandias (ST), Medb (ST) or Quetzalcoatl (ST) for Riders.

In each class, the odds are obviously against you getting what you want, so the obvious question to ask is: Which worst-case scenario would make you least disappointed? If there's a possibility of spending your hard-earned money to end up with Medb or Arjuna, which would you regret more?

Picking to minimize regret is usually the safer option with a guaranteed gacha, unless there's a must-have servant that you don't yet have, in which case, you have to try to get your waifu.

Either. Look at the f2p options for st archer, david euryale billy robin. Look at the aoe riders, medusa blackbeard alexander. Do you have/willing to raise any? Chooe the class you are not willing to compromise with a f2p option.

There's Santa Alter for AOE and i think she's enough as a solid AOE rider. For archer there's Ishtar who can cover both of your AOE and ST roles, at least from the archer department.
If you really want an ST archer, why don't you take a look at Friend Point archers and Robin is more than enough to melt Saber bosses and he has lots of silver ST archer fellow that is as good as he is.
Also Robin and friends are easier and cheaper to raise, unless you have one of your waifu/husbando in that gssr or you don't really care about FP servants, then it's up to you.

Broken servants like Mashu, Hans, Asterios, OG Chu and David is another example of how good FP servant is, especially David he is too good and also he can act as a supp and your main ST Archer.
Good servants always exist.