Unless it stays unlocked a lot of the time for more torque the ratio's of the Allison 1000 leave a lot to be desired. I haven't driven one in years, and never towed heavy with one, but the ratio's aren't all designed for towing. 3rd and 4th are the same as the 68RFE, and 2nd is close, but 1st, 5th, and 6th are geared towards lighter loads. The design is more similar to the Aisin in the C&C, which is a better heavy duty use design and may get more power thru.
There is more to it than simple gearing. Programming with these transmissions is a big deal.
I know my in-laws 06 Dmax will pull the arse out of a mule... without tuning.
The tuning as well as the internal components on the Allison are far superior to the 68.