The shorter you are, the more campsites you will be able to choose from. One of the reasons we love moving down from a 35' FW with kids to our TC without them.
The 35' MH would be bottom on my list since it will limit you to larger roads during the travels. Prevents you from exploring the backroads between the NPs.
Issue with the B+ is they are usually pretty well maxed out weight wise and don't have a lot of towing capacity left. So if you plan on putting a car or a lot of weight in the 20' trailer, you will be lacking power, especially once you start gaining altitude. Another issue I see with a B+ is that is your only transportation unless you plan on putting a motorcycle or smart car in the trailer. Then you have the problem of length when trying to park the car on the same site as the B+ and trailer.
With a 25' FW, you should be able to keep the truck down to a 1/2 ton without issue. Especially if you are a light packer. While it would limit a few places, I would go with a 30' FW and 3/4 or 1 ton truck. This would get you in 90% of the parks which would take you a very long time to explore.
The only exception I have come across with tent sites is with a TC since the foot print is about the same as a truck. This is especially true if you are actually tent camping at a walk in site. Would have a problem with any trailer at these type sites since the parking areas are designed for cars or trucks without trailers.
One option you didn't list is a TC on a 1 ton. At 18-20' with the bed over the cab, not much shorter than the 25' FW and more space than a typical 25' B+. You could still pull the trailer but have the added advantage of dropping the trailer and TC in the campground so just having the empty truck to run around in.
My wife has fallen in love with our TC travel style of exploring all day as we travel and then finding a spot at the end of the day to spend the night. Since we don't have a trailer, we don't have to worry about the road ending as we just turn around. We also don't have to worry about bathroom or restaurants as we always have them with us. And we never have to double back to get to the campsite for the night as it is always where we stop. Sometimes this is the end of the road, sometimes just the parking area for a trail, once in a great while the Walmart parking lot after re-stocking, or that little itty bitty NF campground that is used so little moss almost completely covers the road.
Each unit has it's advantages and drawbacks. The smaller you are the less you can carry but the more places you get to go. The larger you are the more comforts you can take with you but the harder it is to get around between destinations.
So two questions to ask yourself:
1) How much junk/conveniences do I really need
2) Is it all about the destination or is the journey just as important?
2015 Ram 3500 4x4 Crew Cab SRW 6.4 Hemi LB 3.73 (12.4 hand calc avg mpg after 92,000 miles with camper)
2004 Lance 815 (prev: 2004 FW 35'; 1994 TT 30'; Tents)