That's very true. I don't think I would have liked Elantris nearly as much if I read Mistborn first--something about going from a better book to one that is slightly worse, I think. There's definitely something to be said for Elantris being a stand-alone. It's quite refreshing to have one of those with all of the trilogies out there on the fantasy market.
That being said, Elantris is great. You certainly can't go wrong with it. But if your brother-in-law likes a longer read, then Mistborn would be great, too. You can buy the MB1 and MB2 paperback for a mere $13. A surefire way to avoid a lot of these distribution problems for Hero of Ages is to just buy it off of Amazon. You might need to wait a few days, but with distribution issues, it could end up being a lot faster to just wait for the shipping.
Largely, it comes down if you want a long, awesome trilogy or a stand-alone. If you want the trilogy, I can guarantee you Mistborn won't let you down, but if you want a stand-alone, Elantris really is a great novel, too. It comes down to preference.