8 ft v 6ft

Do what you want but the 8' bed has no compromises. I do understand the Mega has more room in the back but the Crew has PLENTY IMHO.

Perfect tow rig IMHO. No compromising.

virtually every choice you make vehicle wise comes with a trade off somewhere, even if it's something as simple as cost. Bigger truck may be better for towing, but not necessarily even possible to park in areas you may want to travel without the trailer.

Most newer 5ers have nose caps, and extended pin boxes that help when towing with a short bed truck. You can still hit the cab though so the responsibility is on you to watch and not let that happen. Bumper pull folks deal with it all the time, although generally the damage isn't quite as much if you jackknife a bumper pull.

Keep in mind that with a short bed truck, and a long pin box 5er you could end up hitting the inside of the tailgate with the back of the pin box if you use a slider hitch, it's another area you have to look in to. In the end, you just have to look at the potential issues you'll run into with whatever combination of truck/trailer/hitch you end up with and make sure you never operate outside the constraints of that combination. Sounds easy enough, but it's also easy to get distracted and have a bad day.

As to the magic answer, in the end it's whatever works best for you. Good luck.

Many newer 5th wheels have contoured sides allowing you to tow with a "short bed". While that works most of the time, get a sliding hitch, considerit insurance!

As long as you stay with a sliding hitch you should be ok. But by getting the Mega Cab you are limiting your towing and payload capabilities. Having the larger cab is nice but the extra weight of the cab translates into less of both of those. Good luck.