I think your upgrade times are compatible at each. Both airlines are adding pilot and airframes. Basing your decision on the upgrade times today in the wrong way to go. When I was hired at ExpressJet, upgrade of Props was 1 year. It took me 1 year 9 month to hold it and I ended up going to Jet Capt before the Prop training slot opened-up. On Sept. 10, 2001, jet upgrade was about 1.5 years! It's now about 4.5 years. What will it be in 4.5 years; who knows.
Take the ExpressJet interview. If they offer you a position, then sit back and weigh your options. I think both airlines will be around for a while and will offer similar pay, benefits, and upgrade times. An advantage to Skywest is that they still have Turbo-props and you'll generally find that upgrade times on Turbo-Props are faster than jets. But then again, who knows.
Be more concerned with which bases better suit you and which company you'd want to spend a career at (you never know). It's a personal choice, good luck. If you get both offers, you're way ahead of the pack.
-minrest.