This is a good question. Binding is very important. I turned to Jeff of Sofawolf for the info, and here's what I've got.
The softcovers are called "PUR" perfect binding, which has to do with the chemical composition of the adhesive. It lays flatter than vinyl acetate and is very durable for standard usage. As with anything, it's not immortal, but it'll hold up as well as any other professionally produced, non-mass-market softcover, with all the usual caveats about not soaking it in water and taking a hammer to the spine and so forth. But it should hold up pretty darn well.
The hardcovers are smyth-sewn, which means the pages are collected in bunches and actually stitched together -- then backed with fabric and adhesive before being bound into the hard covers. This is very durable and considered "library-quality" by some (I get the impression that there are debates over the nature of library bindings, and it is not a settled matter, so we won't get into that--I caught a vague whiff of flame wars past in the air, as one sometimes does.) Anyway, this is uber-durable, give-to-your-heirs stuff--which is why it's more expensive.Either should last a goodly long time, but the hardcover of course is going to last longer, same as any other book.
I hope that helps!
You can order either version of the omnibus through Sofawolf Press.