But the arena fight is low risk, and the ruby quest could take an extended period of time. We don't know the precise location of the ruby, or what stands between it and ourselves.
These, essentially.
Get the ruby. If we get the wrong ruby, we can sell it for stuff, in addition to finding stuff. Also, we can grab everything we find and take back to the shop to sell if it's useless.
Remind me, what ruby have we seen?
None. That's the problem.
Basically: We know where the arena fight is and what the risk is (near zero). We have NO idea where the ruby could be, nor of what danger we will face when we look for it. I'd rather do the arena fight first. We're not exactly going to be facing balrogs, guys.
You can tell from the lack of wings.