The biggest advantage to a motor home that other RV's can't really match is that passengers can travel in the back of a motor home. Since you are traveling alone the majority of the time you won't really get this benefit. I would guess in your situation being able to unhook your tow vehicle and go will probably be a much greater advantage!
As all the previous posters mentioned initial cost and ongoing costs will be higher in a motor home. With a motor home you will have a whole other motor vehicle to maintain on top of the RV maintenance required for a trailer. I can't count the number of people I have known that have purchased used motor homes and have ended up stranded and at the mercy of what ever local mechanic they can find. Maybe this is not an issue if you can afford to buy new?
I have a 26 foot trailer and it suits the 4 of us quite well. If you can make a smaller trailer work for just you I would encourage you to go the smaller route.
All of the used trailers I looked at dealers were a complete RIP-OFF! I bought mine from a private party and was able to afford a much nicer unit then if I had purchased a used unit from an RV dealer.
You might consider looking at a bunk house model trailer. The bunks in mine would be easy to remove to create the office space you are looking for.