While Cam shows up (or not), I’ll give my opinion too.
If you prefer Astram or even if you’re more or less neutral on the matter, I would go with Astram, especially for arena. They both score the same, but Astram is a support unit while also a very strong bruiser. He’s also a GHB unit we just got the rerun for, so he’ll be considerably cheaper. Itsuki is most likely better for AR though.
There’s also many more available dragon killers, even if Itsuki tends to be particularly good at it. There’s been some discussion on the Resplendent Roy thread on whether or not he’s a better choice than Itsuki (mostly considering Grail investment), and for the most part, I’d say he is, unless you particularly like Itsuki.