I ride both, a lot.
Outbound it's probably a tossup--althought there is far more likely to be ice on ARB in the winter on the outbound side. (Not a consideration for now, but ...) You will have more cars passing you at higher speeds and closer to you on ARB than Freeport I think, until you get out where Freeport is wider.
Inbound I would say that I feel Freeport is safer between the Hulton Bridge and Fox Chapel Rd, but then ARB is better inbound from that point to the Highland Park Bridge-and obviously you have to commit to one or the other at the Hulton Bridge. The traffic is closer to you on ARB and I have to say I don't find it all that much slower, but inbound you don't have cars darting in and out of parking lots and streets etc. by the Waterworks and through Aspinwall on Freeport.
I don't know about the trail in Oakmont, but we always swing across the tracks onto Allegheny Ave even if our destination is up Hulton Road. More room and better road surface.