Ken used both types of pants during his recovery. For lounging around the house, he generally wore either his bathrobe with nothing underneath, or a pair of track pants with an elastic waist. He also used track pants for his daily walks.
However, we were up and out after a few days several times to go out to eat, do some shopping, go to a movie, etc. He did wear loose in the waist "regular" pants for outings, and he did not find that they irritated his wounds for these type of activities (although he found elastic track pants more comfortable for his walking).
Since you will be in a motel for a week, and if your recovery is going well, I imagine you will be getting stir-crazy after a few days and will want to go out to eat and so forth. Might want to try your regular pants in a size bigger for these type of outings, instead of buying new ones that you will never wear again? If it does irritate your wounds, I wonder if putting like a gauze abdominal pad over the belly wound would help? (don't keep it on, the wounds need to breathe, but just if you were wearing regular pants out for dinner or whatever).
You could always try your regular pants a size bigger, and if they don't work make a trip to wal-mart for some stylish elastic pants :)