If you are just flying between DFW and HOU then probably no major downside as long as you are wanting to go to that part of Houston. They only use the CRJ-900 so if it isn't your cup of tea you are out of luck there. If you are connecting through DFW to HOU then the downside would be that DFW is your only AA option. If things in DFW get behind for weather you might be there for a bit whereas you have many more connecting options out of IAH. I've done it before because I was visiting a client on the SW side of town so it was a bit more convenient. I also got upgraded as a Platinum whereas I don't think I have ever been upgraded to IAH.