You can carry more with the 6T becuae of the lighter weight of the car but tow more with the XR8 but AUIIIXR6VCT is probably right about the XLS V8 which has the best grunt and a set of springs that is not set up for 500kgs max. Apparently the SOHC V8 has the best low down torque of all the engines and with 220kw it is not going to be too slow by any means.
Probably not what you wanted to hear is it. The other thing is I am not sure if the 6T would produce the power where you want it (2000rpm+), is perhaps just a little too high if you are traveling with a trailor.
--Lowndes, Bargs and Ingall, all got sick of promoting an inferior product--