Aside from the points made above, the main consideration for us was: are we traveling or are we staying? A C is for traveling in our opinion. If we are staying somewhere we want the trailer. A trailer for me is not for moving every day or even every other day, a C is. For us, a C did not make sense if we are spending a week or more in one area. We want the extra room, comfort, and storage of a trailer.
Of course a lot depends on how big a trailer you are talking about versus how big of a C. A big C will have more room than a small trailer, but for a set price you will get more space with a trailer.
A $15000 C is going to be pretty old and a similar priced and sized trailer will be relatively new. The trailer will need a Tow vehicle and the C will need a vehicle to tow.