Late reply, I just recently joined, but in case you're still in the market buy the FBS model, there is no comparison how much more room you have with the slide out.
I also tow with a 2007 Nissan Frontier CC 4X4. I currently own a 2010 Shadow Cruiser 18fbs. Frontier is more than capable to handle the extra 300lbs that you were concerned with. See my other posts, I have pictures posted where it shows additional modifications I have done to the trailer adding more weight and the truck handled it fine.
Infact the trip with my Yamaha YFZ 450 in the back I have posted the best average MPG....strange but true.
By the way I paid $16,000.00 for it
Nissan Frontier, 2010 Shadow Cruiser 18fbs. Honda EU2000ia