A few observations:
1. Being a "dually" doesn't guarantee it can tow any 5W you might like, so buying the truck first automatically places some limit on your ultimate choice of trailer. You need to look carefully at the truck's tow rating and Payload (cargo carrying) capacity and compare that to typical weights for the size & style 5W you like. Both trailer gross weight (GVWR) and pin weight (the weight that falls directly on the truck).
My Ram 3500 dually has a gross combined weight rating of 39,100 lbs leaving me with almost exactly 30,000 lbs of towing capacity. In addition to the 5,680 lbs of payload, what fifth wheel out there CAN'T it tow?
2. A dually can generally handle more pin weight because the dual tire axle is rated higher than a single (called an SRW - Single Rear Wheels). The overall towing capacity (tow rating), however, will be about the same as an SRW. Since a 5W puts a lot of weight on its hitch pin, this can be a very important consideration. However, you mus check the actual numbers (see #1).
According to Ram, the max trailer weight for a SRW version of my truck is 16,860 lbs. That's just slightly more than half of what the DRW version will tow. How is that about the same?
3. You probably can't have "too much truck" when actually towing, but you surely can at the shopping mall or grocery store (whether you are "feeble" or not). Any truck that can tow a substantial 5W is going to be large, and making it a dually makes it even larger (in width)
4. Traditional wisdom is that a dually is more stable on the road, i.e. the extra two tires give more grip on the pavement. I'm not very impressed with that argument and nobody has any real data comparing two trucks that are identical except for SRW vs dually axle. People move up to a dually from a half ton and think Wow!, this is really great, but much of the difference is that they simply have a bigger, more capable truck. A truck that is sized adequately for the trailer will be plenty stable and won't get pushed around willy-nilly by semi's or crosswinds, regardless of the number of tires. Plus, there are situations where dual tires actually have less grip, e.g. they may spin more easily in mud or sand. I'm not arguing against the dually, but I am down-playing this supposed advantage.