[ytse43]

(I own an FS5.)

The FG840 has flamed maple back and sides, the FG3 has solid mahogany b/s--the same as the CSF3M. The FG840 has a 650mm scale length, the FG3 634mm. (600mm for the CSF--for completeness!) The top on the FG3 has undergone the A.R.E. process. All models have scalloped bracing.

You want:
A) Resonance I can feel in my hands
B) Durability
C) Balanced sound, with healthy low-end punch.

A) When I first play-tested a Red Label, this is what hooked me. The guitar vibrated. I got goosebumps. I also played a CSF during the same session. I liked it a lot, as well, but knew I wanted a slightly larger body. (This model does have its place, and I wouldn't mind having one.)
B) This is YAMAHA we're discussing, 'nuff said. (That said, the semi-gloss finish on the Red Label series is perhaps more finicky.)
C) I can't speak for the FG840, but the Ibanez PF40 I've had for well over 20 years is also a dreadnought with flamed maple b/s, and it have never had a "healthy low end." The FS5 does. It definitely punches above its size. (I play finger style, and also wanted a smaller body, hence not going for the FG3/5.)

Hope this helps.