If you're in a position to get the 16" rims and 6000lb axles, I would do it in a heartbeat!!! No matter how folks try to justify it, that size and weight of trailer with just normal loading is going to max out or be very close to the max rating of the average 15" ST D load rated tire. Why not get the extra piece of mind of having a little extra carry capacity and not have to worry about the 15" tire loading ( blow out) issues that so many people are experiencing with 15" ST tires. Many blow-outs are being blamed on the china bomb tires, while there is no question in my mind the mainland chinese tires are very suspect in quality, the biggest issue that causes most tires to fail is heat. A tire going down an already hot road at or near its weight capacity is just a time bomb ready to explode.
I would definetly go with the 16" / 6000lb axles on that model. Yes, you will still need to replace the OEM China Bombs, but you can replace them with quality 16" LT or ST series tires. If you don't, I promise you at some point in the future, you'll wish you had!