The T1 is hard anodized which gives it a more rugged, matte finish, it has night vision compatibility (meaning it has some extra low brightness settings and the lens coatings are designed to pass longer wavelengths) and a Picatinny rail interface.
The H1 has standard anodizing so it's more of a satin finish with a tad less resistance to scratching, no NV compatibility features, and a Weaver interface.
In use, unless you want to use it with a night vision device, you won't notice any difference other than the texture of the anodizing and the fact that the lowest brightness settings are usually too low to ever see the dot. I've used them both on the same rails, and I'd take either one. The T1's hard anodized finish looks nicer to my eyes however.
As for price, you'd have to ask Aimpoint. These sights are of very high quality and ruggedness, and they're so efficient with the power that even the little button cells they use get years of constant run time on a single cell.
The downsides: they come only with bikini style lens covers. Other companies, like GG&G for example, are making solutions for that. The Micros don't accept the KillFlash anti-reflection devices.
I'd say we sell about one KillFlash to 10 or so of the regular Comp sights. Something like that, so the lack of that option wouldn't be an issue for most people.
The main reason to go with a Micro for me is weight. Even with the supplied mount, they're several ounces lighter than the regular Comp sights alone, and of course they take up less room. A secondary reason is that they sit just a little bit lower on the rail, and that's a benefit on some weapons, like AKs, M-14s, Mini-14s, and M1 Carbines. On an AR this matters not at all, since you have to add height either way.