We are very happy with our 120 watt portable suitcase unit -- we have attached a 30 foot anti-theft cable from Harbor Freight, and it is fairly easy to carry the panel on the bed in the TT while traveling. Your tow vehicle charges the battery while towing (not as well as a solar panel, admittedly). But the advantage is that if you do not plan to keep the trailer, you can keep the portable panel.
The other advantage of portable is that it is tiltable and can be pointed at the sun. The flat panels can't.
But having said all of that, if I had a lot of extra real estate on top of my trailer, I would plaster it with panels. Set it and forget it! The portable has to be deployed and stored.
Life is a series of compromises.