I will give a bit more info....
I had a 1991 80 with a 5.7L vortec from a 97 Tahoe. Cali legal with the sticker. 4L60e GM trans to FJ60 split case. 35's on it and Slee HD coils all around and Fox remote resi shocks. I converted the steering to GM 1-ton and got it in front of the axle. I cut & turned the front axle setting caster at 4.5* rather than use bolt in caster correction. I custom fit a King steering damper. I did a number of road trips in this truck, including down to Baja. I DD this truck about 2yrs and did some decent off-roading in it. Once I did all these mods, it easily drove as good as the stock 100 I just bought.
Just picked up a 2000 100 and drove it 1500 miles home. It is all stock. It is nice, handles good and power is not exciting but adequate. It is obviously a bit more comfy than an 80. Just got 14.3 to 17.3mpg on the 1500 mile trip.
If you want to off-road a lot the 80 with the v8 is better. No doubt about that. With the FJ60 t-case you will be part time 4wd. Believe it or not the 97 vortec is actually better than the 2UZ. More power down low, much cheaper to work on or upgrade and got slightly better mpg. The 2UZ is smoother that is for sure. Other than the timing belt every 90k it will likely outlast the vortec. At 80mph in the 100 I was getting 14.3mpg. At 80 mph in the 80 with the vortec I got a solid 15+mpg. At 80mph in the 100 I am at roughly 500-700rpms higher than I was in the 80. The 80 had 4.10 gears FYI. The 4L60e was no worse of a trans, mine had a shift kit on it and engine/trans tune to match. It was excellent. Nobody can argue the FJ60 split case is good and strong and easy to work on even if a bit loud. I have done a sand dunes trip with a buddy who had a 100. Side by side the 80 with the vortec did better in the sand, not a lot better but enough to notice. He drove mine and I drove his to compare and we both agreed the 80 with the vortec was better.
I have already thought about doing a SAS on the 100. It is intriguing that is for sure. But a lot of work and money. I don't think an 80 axle will just go right in, as shown above the 100 frame is wider up front. Maybe easier to do with coilovers? If you just swap in the 80 axle you are left with 6 lug front and 5 lug rear. That is no good and must be dealt with. I suppose you could swap in an 80 rear axle too but aren't they a bit narrow in the 100? I haven't put a tape on either yet to see.
Unless you won't drive anything but a 100 I think the 80 with the GM v8 is a better all around option. They have everything the 100 does besides the heated seats and rear a/c. I think Georg at Valley Hybrids is doing the older GM v8's like the vortecs in an 80 for about $15k and that is Cali legal. You could get it done for under $10k if you did it yourself and if you don't have to deal with Cali rules you could easily go with a much better v8 like the 6L. Looking like $10k+ to do a SAS in a 100 if you do it all yourself. Perhaps more than double that to pay a shop and go with a custom axle. Good 80's can be found for $4-5k with locked axles. 2nd hand GM v8's are cheap. Good 100's are running $10k+. So all in all it may be comparable money wise with the 100 SAS being a bit more perhaps. You can see by the pic above of the frames the 100 is flatter above the front axle so obviously this will limit your up travel compared to an 80. Otherwise if they are both radius arm they will perform the same.
it is very temping for me to follow through more with the SAS swap in the 100 simply because I got real nice locked 80 axles and suspension just sitting here plus some other goodies I could toss at the build. If I didn't have plans for the axles I would probably pursue it more but I think they are better suited under an FJ62 I have. All in all it is your truck, do what makes you happy with it. I do think it is comparable to put a v8 in an 80 and build it up when it comes down to the $$$ you would have in each truck. Some people though will never drive a Toyota with a GM engine in it though. Some people will never give up their 100 for an 80. I can tell ya from experience though all the 100 has on the 80 is more comfort and bling. Otherwise, comparing the two, a v8 80 is very similar to the 100.
Cheers
