I find the IIF viewfinder to be pretty squinty. It is servicable, but not the greatest.
The IIIG finder is really nice, bright, and I love the parallax correction. I wear glasses, and I can see all of the frame lines easily. The 1.5x magnified rangefinder is also great for accurate focusing. In fact, only the M3 and 0.85X finder Ms offer a better or similar focusing effective baselength. The 0.72X M finders have a shorter effective base length. The RF patch may be dim on these older cameras though. I've had DAG replace the beamsplitter on mine - now it's really bright.
I use my IIIG with the CV Heliar 50 f/3.5 which is really, really sharp, and also the CV 35 2.5, which is a tiny and very handy lens (also very contrasty). I do need an external finder for the 35mm, but it's not a stretch to go from the RF to the 35mm viewfinder.
I've just obtained a M7 0.85X finder. The first thing I realized is that I can't see all of the 35mm frame lines with glasses on. However, the view through that coated M finder is astonishingly clear and contrasty! It's now at Leica USA getting a DX reader upgrade and MP finder upgrade, so I can't say more about it. However, my first impression is that the IIIG is a lot smaller and lighter than the M7. The IIF is even lighter, if you can live with that 50mm only finder.