The sad thing about tools these days is "professional" basically means "home" and home tools aren't made to be run the 90 hrs it might take you to polish your trailer. You are going to have to keep your polisher clean of debris to keep it working, this means blowing it out with air and the occasional disassembly. Even keeping it clean may not be enough to keep your polisher in working order.
If you buy the dewalt you are buying a tool with replaceable parts that any reputable repair shop will be able to fix. Prices here were about 25 for new brusshes, 40 for a burnt out cord and 40 to replace a broken handle(whoops).
If you buy the Harbor Freight you could get lucky it might make it through your final coat or you might burn through two or three of them. Harbor freight has as i understand it' a ridiculously liberal insurance/return policy i know of a few folks who abuse it readily.
I got my Harbor Freight when my Dewalt cut out on Friday and i couldn't get it into hellgate tool untill monday. The one i bought is much louder than the dewalt and has a finicky 1-7 speed control (the dewalt has rmp values). I like the dewalt, it feels nicer in my hands. The Harbor Freight has worked for me every time...every time the dewalt was in the shop.
metabo makes a polisher for 260 it lasted a trailer and a half for me, but this was before i realized the importance of maintenance on these "professional" tools. Even though the thing died on me i would consider buying another because of the amazing ergonomics of the thing. Very easy to get your whole body into controlling the thing.
MOST importantly for all of your tools esp. one that gets run like a polisher use a good thick cord and make sure you have the (redundant) ground.