Ughh.....that's a tough debate. The miles are really the least of your concerns, especially given the fact that both have the same drivetrain.
Underbody storage can be fabbed up easy enough though even sheetmetal is somewhat expensive these days. Having it premade would be great. Having to work around it for your batteries, tanks, etc wouldn't be. I'm not really sure which one is of bigger benefit.
I would love to have air ride, especially if I put more miles on. It iwll be one more thing to maintain. Basically, if you plan to drive long distances at a time or put a lot of miles on in a year I would go with the air ride. If you won't be touring all that much due to other employment and whatnot, the spring suspension might be better for you. Certainly I would think the air ride would add some resale value, not that our buses have resale value.
Tires can be cheap. I would see no problem running retreads on the back (NOT the front). You could also run bias ply tires at about $150 each though radial tires are better. I have a soft spot in my heart for bias ply tires because that's all my truck has had for years. I'm used to square tires on a cold morning.
Getting the tires installed will take time and depending on where it's done, may be expensive. Prices seem to run all over the board on that. I paid $75 each to get my front tires install new. In Bloomer, Wisconsin I paid $10 (plus a $20 tip cause the kid was cool) to get a used tire mounted.
Look at both VERY closely. Give them both a good inspection and look for problems that you can use as a bargaining chip. I've never dealt with Greg, but maybe he is willing to come down in price a little. Again...rust never sleeps.
No matter what you choose, I'm sure you'll be happy. Let us know what you do. Unlike some other sites, we DO like "baby pictures" of your unconverted bus. Even the driver's console.