For the sake of posterity:
If I would have bought one, it most definitely would have been the twin. Marginal price/weight difference, same footprint, more space, especially in the max-headroom-area category.
Joe at zpacks said, "At 6'3" you are pushing the limits of the tent. The Solo-Plus size has the most space above your head and feet. Normally there will be enough space that your bag won't touch, but if it is windy that will blow the walls in enough to touch your gear probably. One thing that can help with that is if you prop up the sides like this:
So, I took that as not what I wanted. Joe was kind enough to offer to play with the design to accommodate me the taller person, but I needed something a bit quicker, and more resellable.
I started taking a long hard look at the lighthearted solong 6. It's 27 ounces by default un-seem-sealed. I thought, gah, why? It's like 27 ounces. I'll just go with a hexamid with a beak/netting for like 11. Well, it turns out I bleep'ing hate my feet and head touching. I enjoyed how light my poncho-tarp/bivy setup was, but man, when I wore out my knee on the JMT last year I didn't really love having to lift it into my wiggle bivy. I didn't really love not being able to sit up and read, or move around and be able to have my breath not condense to the side/floor. I hated having to retreat from mosquitoes into such a cramped space. So, in the end, it's worth an extra 12 ounces (well more including a new umbrella) over the poncho/bivy for me to have a tent I don't have to wiggle into and/or have my head/feet touch the mesh on. This isn't even to mention the huge cost of getting a solo+ + beak + cuben floor + sewn in custom option? + the whole shebang. My wanted setup seemed to ramp to about $600 in a hurry.
I also took a look at some tarptents, including the notch which Henry thought would work, but leave me touching the mesh with my feet. He said that wouldn't be a big deal, but, after having done this for years, I didn't want that. He recommended the The Rainbow and StratoSpire 1 as a better fit, but said it'd be added weight.
So, yay! A lighthearted solong 6. $300 spent. I get to stretch out, which is pretty much the most exciting bit. 26 ounces. One happy dude.