The AG doesn't argue cases before SCOTUS - that's the job of the Solicitor General. For the AG, it is incredibly important that he be familiar with the organization and workings of the DOJ so as to quickly clean house.
Sessions has been on the Judiciary committee, and his conservative bona fides there have been unassailable. I can't think of a single issue that will be within the scope of DOJ on which he has been on the wrong side. Civil rights, immigration...everything.
Moreover, he's been a state AG, so we know he has the organizational skills to run an entire department. And he himself has been a U.S. Attorney, who are the DOJ's front line troops.
If there are others who would have been a better choice, that's a very short list. Neither Cruz nor Gowdy was either a U.S Attorney or state AG. I think either would have been great choices as well, but it's hard to find fault with the pick of Sessions.
In any case, remember the core metric - how does Sessions compare to who Hillary would have appointed? For those of us worried that Trump might not be better than Hillary, this is an absolute home run pick.