Doing the same thing; just placed an order for a Yamaha ef2000is last night. I went with the Yamaha due to quality, portability, and the ability to move to a parallel setup if needed. Wife and I went to Cabela's last weekend; they had a Champion and Yamaha sitting right next to each other. Just playing with the switches, pull-cord, ... IMO you could tell the Yamaha was many steps above the Champion in terms of quality.
Only have heard these generators running in videos, but based on that it just seemed the Yamaha engine had a nice even purr vs a semi-stuttering sound of the Champion. Semi-stutter is probably too strong of words, but just seemed the Yamaha's engine sounded like it was under less stress too me in the videos I watched. Plus, I will be storing my trailer in a storage facility that does not have electric available, wanted something I could easily throw in the back of the pickup and run up to the storage facility to perform maintenance, etc.. Yamaha is easily handled with one-hand.
In terms of the adapter; I am thinking this
15-30 amp pigtail adapter. That way I should be able to connect directly to the generator; thinking the normal 15-30 amp adapter would have too big of footprint (e.g. would take up both plugs or too big period).