I used to have a 1292Q and I've had 2 FP screens. My first one was 120" 4:3 1.3 gain screen, my second one was a 138" 16:9 2.1 gain screen.
I had gotten my first screen with the same points that you mentioned in mind, burn-in, and a larger 4:3 picture. I didn't mind the letterbox bars because they were not really visible in a darkened room, and it was either going to be letterboxing (with a 4:3 screen) or smaller letterboxing and sideboxing (for 2.35:1 and 4:3 movies on a 16:9 screen).
But I wanted to be ready for HDTV which is natively 16:9, and discovered that the majority of my projector watching was widescreen movies, plus there was ultimately a height limitation for the room. Plus I like the look of a 16:9 screen better as it looks a bit more like a theater screen does, rather than a large 4:3 TV.
Now I have a digital projector and a new 16:9 screen for the same reasons as before.
The biggest determining factor will probably be which video format do you watch most, and what format are you planning on watching the most in the future.
Another factor may be the price as the taller RP screen might cost more money. Or not.
Either way, I'm sure that you will enjoy your new HT whichever screen format choice that you make.
My 1292Q had a stunning picture, especially with scaled DVDs and HD material.
You should do a search on RP screens here as many different RP screen materials have been discussed in the past with regard to cost, viewing angle, gain, and resolution capability.