I too am a female rider with decades of riding 2-wheelers who has recently switched to a Spyder. I chose the RT after originally planning to get the F3-T for several reasons, but possibly most pertinent to you is that, I too have a bad back (from a horse wreck years ago) and I like my feet to be straight under me. Granted, I'm used to that position from horsebacking, MCing, and ATVing, but I also feel that when my legs are angled forward, as the other models require, it tires my back. AND, with your feet more in line with your hips, you can more easily raise your weight out of the seat to reduce spinal shock when hitting bumps or potholes.
Also, don't be too quick to dismiss the cargo capacity of the RTs. You may very well decide that it's a blast to go shopping on your Spyder. And, although I originally considered the passenger seat to be extraneous, I now realize that it offers all sorts of perks, like strapping a sack of horse grain onto it or a large purchase that juuuust doesn't fit into the frunk or simply that many more groceries.
In short, my admittedly newbie opinion is that the RT is very comfortable, lets you rise into the "two point" position, hugs the corners just fine!!, and allows more fun stuff to be done with it.