I haven’t commuted between hubs before, but I have deadheaded several times between IAH/ORD/EWR on 737s. Every time, the flights were 100% full, with 10+ on standby. There’s usually other pilots standing around but I know there’s not enough seats for them all. I don’t see how guys can commute on such busy flights all the time. I suspect something like SEA-ORD will be packed full all the time, but it’s worth researching. Passrider.com is one site that will help you with commutability.

That said, my ideal commute is hub to outstation on RJs where the city is large enough to have reliable 121 service, yet not so large that flights frequently become oversold. SEA-MCI or IND isn’t ideal. But I would probably be choosing this over the hub to hub routes I mentioned earlier.