-Spd/+Res vs +Atk/-Res Corrin

Speed is the worst bane you could get for her. She gets a lot of speed from her weapon, but with a speed bane she isn't fast enough to double consistently. I would recommend the +atk/-res one.

I managed to pull a +Atk / -Spd and used her this week for Arena, and I honestly don't think -Spd is that bad of a bane. 34 Spd may seem unimpressive at first, but the +10 from her weapon and Atk/Spd Bond brings that up to 44, which is good enough to avoid doubles from even +10 Spd demons. At that point, you can have a teammate provide something like Spur / Drive Spd or Hone Spd / Spd Tactic to double consistently on both Phases, or you could give her the Darting Stance seal to double more reliably on Enemy Phase. (This is even easier for now since she currently gets +4 to all stats in the Arena.) Personally, my team does have Drive Spd support, and I don't think I ever ran into any units that Adrift F!Corrin couldn't double on either Phase.

Now, I'm not saying that Spd is an ideal bane for her, but I think that it's a more workable bane than -Res is, especially since Res is a superbane. (Dragons are still very common, and -Res is a huge detriment in her ability to tank them.) If you are sticking with the default kit, I do think that you should go with +Res / -Spd.

Some weeks ago I shared this line that I'm told my grandfather used to say, and I'll share it again: "Rich people don't think about how to save money; they think about how to make money." My point is that people tend to fuss about the bane stat a colossal amount, when in fact I'd say it's better to focus on the boon stat at least as much if not more. It's worth to count the cost, but more important than the cost is what you're going for.

Having said that, I refer you to my response to another F.Azurcorrin question here https://fireemblem.gamepress.gg/q-a/aaaaaaaaaaaaaaaaaaaaaaa

I personally definitely don't like the res bane, but she does fill a different role better using that IV than +res -spd.