I have a Radian RF in the center of our 2008 Prius. I'm pretty much at peace with the install, although it is not perfect. Front seats have to be a bit further forward than we like, and the Radian does brush the front seats when DD sits in it. Prius manual says not to touch at all, but we've decided that a fairly light touch is unlikely enough to actually cause problems that we are willing to live with it.
As mentioned by LISmama810, Ketchupqueen has several posts about installing the Radian in a Prius. I would add the caveat that her installs were (as I recall) fairly short-term, in a rental car. I was able to get our Radian more upright also initially, but over a week or so, with DD riding in it, and standing up in it after unbuckling when we get home, etc., it would always settle back into a more stable position (i.e. more reclined).
I was able to get a very solid install, with some work, and twisting the buckle stalk twice to get the buckle away from the belt path. I ended up using a locking clip (using the locking function on the shoulder belt made the seat tilt, and also pulled so hard on the plastic trim piece -- where the shoulder belt comes out of the vehicle backrest -- that the trim piece was pulled out, which looked ugly). This is just to say that I do know how to get a tight install with the Radian, and still couldn't get it to stay SIGNIFICANTLY more upright long-term in our Prius. Somewhat more upright, yes, but not a lot.
I didn't try as hard RF outboard, but did experiment with it. Again, got a tight install, but the Radian 'wanted' to settle back into a more reclined position. Outboard I experimented with moving the boot forward from the seat bight (there a little roll near the back of the vehicle seat cushion that seems to invite this) and got a nice solid upright install -- met all the criteria of less than an inch of motion side to side and front to back (way less than an inch, actually). However, I then wiggled the seat fairly vigorously, simulating what DD would do climbing into and out of the seat, standing up in it to look over into the front vehicle seats, etc., and fairly soon the boot shifted back up into the seat bight, the Radian tilted back considerably, and the seat belt was now loose. I'm not willing to check and reinstall a seat on a daily, or even weekly, basis, so I decided this was not an acceptable install long-term.
I don't remember how a solid outboard RF install, with the boot back in its stable position at the bight, compared in 'uprightness' to the center install; we prefer the center install for other reasons, so I just moved the Radian back there after deciding the really upright outboard install was a no-go.
You mentioned that you were thinking of perhaps FF one of your Radians. FYI, I do have a Radian installed FF outboard, driver's side, and it is very solid. Again, it was not an easy install, but can definitely be done (recline 'foot' down).
The Prius is pretty roomy in terms of leg room for the back seat passengers, so if you're willing to more the front vehicle seats up enough, you may do OK with 2 RF Radians.
I don't know how installs in the 2010 (Gen 3) may differ from our 2008 (Gen 2) Prius -- don't know which model you are considering.
Katrin
ETA: All the installs I described were with seatbelt; didn't try LATCH outboard, mainly because I needed to get three across. Center install in the Prius must be done with seatbelt (no LATCH anchors in the center).