Hello Greg,
I haven't been on in weeks and saw your question, here's my 2 cent YMMV of course:
1. Other than weight are there any big benefits of one vs the other?
On the GFX100S no accessory grip, you lose the tilt and swivel EVF, lower resolution EVF 3.69mp
On the GFX100 you have a grip always - although it is not the best feeling, you get the tilt-swivel EVF and higher resolution EVF 5.76mp
In reality the EVF difference is a lot less than I thought, although I missed the tilt and swivel - although that was an extra piece of hardware on the GFX100 that made it even bigger and heavier
On the GFX100s you use the new battery NP-W235, which has a great dual charger option from Fuji and is 2200mAh at 7.2v, the NP-T125 is 1250mAh at 10.8v so the newer battery for the GFX100s has more capacity, the GFX100 can hold two batteries and the GFX100S can only hold one
The GFX100 is much larger and heavier
There are some user interface differences, I prefer the 100S, however that is very personal
Image quality is identical from 1,000s of images on both
2. One of the reasons that the GFX interest me is that I have a some Hasselblad H lenses and Leica S lenses and it appears there are adaptors for these. Anyone have experience with adapting these lenses?
I've used the Hasselblad adapter (Fuji as you know co-designed the H-series lenses) and it worked fine, albeit slower focus. I didn't try the S adaptor since much of the time my S-lenses were on holiday in the EU getting fixed (ultimately that got resolved and the last fix was permanent, however at that time I moved from the S007 to the SL2).
3. How do these cameras perform handheld?
Again YMMV, I didn't like using the GFX100 handheld (at all), I totally enjoy using the GFX100S handheld, the IBIS on both work well and they are both fine on a tripod. I would say I use the GFX100S about 3x more often than I did the GFX100 due to size/weight.
4. Best wide lens?
If you want to stick with Fuji you currently have the 23mm (18mm equivalent), it is a really nice lens. A 20-35mm Zoom is in the works and will most likely be announced in May and ship late this year, although it could slip to next year. My basic kit is the 23, 30, 45 and 110 if I want only primes, which is often the case for me. The 50 is an amazing lens creating a 100mp compact point and shoot for walking around
The zooms are a bit more complex - the 32-64 is great and the new 35-70 is great (and might even be a bit better than the 32-64, it is tiny although a slow max aperture). The 45-100 is excellent and the 100-200 is not as bad as often reported, its actually fine. The 250 is a tank but stellar even with the teleconverter. I have never used the 63 or 80. There is also a tilt-shift lens in the works and a few others on the roadmap. Why Fuji used so many filters sizes is annoying though.
Any additional ideas or feedback would be appreciated!
Where I ended up on this journey was H3Dxxx to S007 to X1Dii to GFX100 to GFX100s - that was a 2 decade journey. The S007 is a special system as you know. The GFX100 was purchased used and was the most short-lived (3 months for me, I just didn't get along with it), the X1Dii/907x was about keeping my V-system alive - a whole different journey since I had V-system lenses and bodies going back to the mid-80s. While I loved the S007 optical finder and the beautiful lenses there were just too many issues and the upgrade path to the S3 didn't make sense for my purposes. The SL2 and the L-glass is stellar (along with a safe bet that the SL3 is going to be 60mp) and the SL system gets used a whole lot more than the S007 ever did. The GFX100S is my choice for serious landscape work and serious road trips.
There are a lot of L-brackets for the GFX100S, I found the Kirk to be the overall most usable.
Hope this helps in some small way, Glenn