Guaranteed SR

is still very far, wait a few weeks before the banner to ask this question, you're still going to get a lot of servants until then.
if you find grammatical errors blame google translate

Atalanta isnt that great without skadi, her np gain is pretty bad sadly. Gil is a great support that enable arts critics team, instead of tamamo who enable more art spam teams. I'd say pick gil for now.

Depending on what team you want. Atalanta = Skadi quick NP looping. C Gill = Arts Crits.

So a few things of note:

** Both are non-limited 4*s that could theoretically show up at anytime over the next 8+ months so I wouldnt get hung up on just them as you may find something else a bit more appealing/useful when the time comes **

Now to answer a bit of the question: I have no experience with Cas-Gil other than the story, however I am of the Pro-Atalante and Pro-Quick camp but its not without its demerits.

1) Quick in general kinda sucks (damage wise anyway) until Skadi arrives and even then shes only a band-aid. While I would say at present Atalante is kind of a pseudo support, she doesnt give the general quick team the one thing it needs most, survival.

2) Archer star weight and abysmal NP gain. While her NP brings good damage and a shit ton of stars, it doesnt change the fact that youll be lucky if you pull it once without a K-Scope which is unfortunately not the type of CE you wanna run in a quick/crit comp. Furthermore she sucks the stars away from your real damage dealers (Scathach/ Okita) and doesnt allow them to crit or build NP the way they should be since they are the ones that will be doing all your damage not her.

3) Quick is expensive as fuck. Unless you're lucky as hell or plan on taking a mortgage out for this game, the servants you need for hard content are limited and 5* (save for possibly her and Jack). I got kind of lucky and plan on pulling for Dantes and Okita to round out the Quick All-Stars, but you pretty much need to have at absolute minimum 1 of the trinity (Jack, Okita and Scathach) but preferably all of them as well as some pseudo supports you can swap in and out as well as some high value CEs.
**Spoiler: Skadi is also limited and also 5* and is basically a necessity to make the card type work in the future, so theres also that.**

Pros:

1) Incredibly fun to play and Atalante slots in nicely as a buffer/backup damage dealer as long as you can get her NP off and get the stars away from her to someone like Scathach or Okita and let them do their single target Nukes for absurd damage, pour the stars out and do it again a second time.

2) She has a berserker alt in JP that is honestly better than her. Now normally given the already bad survival that Quick has I wouldnt slot a berserker in there, but her alt fixes all the issues her normal has. Good NP gain, Same solid stargen and almost identical skill set so she still buffs all the quick servants without pulling all the stars for herself. Win-Win. **Bonus: shes a berserker so she hits like a berserker so theres that.

All in all, I like her, but she does little for a team that isnt completely quick based and unless you plan on rolling all your resources into that or already have all the necessary pieces to make it work, your money is better spent elsewhere, but as everyone always says, if you like em, get em since you dont need any of them to actually play the game. Hope that helps some :)

Out of those two, I favour Gil since Atalante's stargen mostly comes from her NP which she really needs help to charge, and when it comes to stargen there are other, better options, but tbh both are fine.