I'm not sure how Cleveland compares to Philly, but I find 28s ample for most riding here - and I just avoid/slow down for the potholes. I have two bikes I use the most for commuting - one a MTB and the other a tourer. The MTB got the mods you'd expect - drops, a rack, slicks. The tourer has more of a sports tourer build as far as components, but with hardcore racks.

For a longer commute, or a ride after the commute, I prefer the tourer. I like the MTB more for gravel and bad weather. I also like that it's a ti frame and I don't have to worry about corrosion or paint chips at the rack. They're both wonderful bikes, but I find the tourer more comfy over distance...and more stable with weight. Like lester, I really prefer a lighter MTB frame. Since MTBs are usually a song on the used market, it's not that hard to find one. Also in the MTBs favor - it's probably less than a third of the cost on total build.

I have not found that MTB tires flat less than thinner tires - I've found that it's the tire used more than the size. If you're hitting potholes at speed, obviously more air in a thicker tire will help, but I just avoid hitting the nasty ones at speed. If anything I flat less with a thinner tire because there's less contact point to pick up debris.