Not an expert by any means, but what my trans guy has told me.... The 5speed is not quite as robust at the 06 and newer 6 speed that came with the last LLY and LBZ. 6th is for freeway cruising ,, you don't tow in 6th for engine RPM torque range reasons. I can not offer anything about the strength of 6th should you do a gear ratio change,, but you have no business pulling a trailer at 80MPH anyways. So RPM / Gear / and speed.. it just makes sense that your not towing in 6th so..... Is a 5spd 'good enough' ??? Again from what I've been told by my trans guy.. the 5 spd is not as robust at the 6speed. When the 6speed came out with the LBZ (with a SIGNIFICANTLY higher torque output) was available. I don't know which, if any hard parts were actually updated, or if it was just machined / fitted with more clutches. BUT as I understand it, the trans was updated to handle the added torque of the LBZ, and one of the suspect components of the 02-05 5spd is the early convertor. If you pull heavy and hard, you will soon find out why the triple disk / billet stator / billet cover convertor is such a popular upgrade to ANY Allison.
As a FYI, you can do a valve body mod to the early 5Spd and make it a 6 forward gears, but don't confuse that with the later updated 6spd,, the trans was updated for the higher torque. You can build the 5spd to handle about anything you throw behind it or in front of it. It may be worth considering if you find a clean pre-06 you like. $ for $ older truck with 5Spd and a trans build may be cheaper than newer truck value .
The discussion above covered it well. The key points of 5 vs. 6 when towing is that it doesn't matter because you're not going to use 6th (and often not 5th) when towing. I think the 5 speeds are just fine, and well matched to the Duramax (LLY for 2004.5-2005) as long as you don't add a tuner.
I added a tuner to my 2005 Duramax as well as the larger transmission oil pan & a larger Tranny Cooler. When I want to tow moderate to heavy loads, I just borrow my dads 2011 LML 1 Ton. Until I do a tranny re-build, I watch what I tow & am careful to watch the tranny & engine temps & not push things. There is a huge difference between what i can tow comfortably with my 05 LLY & the old guys 1 Ton.