Bottom line: the 100 does virtually EVERYTHING better than an 80. Now, keep in mind, I have an 80 and love it. Don't plan on ever getting rid of it, because there are things it does quite well (very stable, easy to set up for off-road, I personally like the way it looks better than a 100, etc.). That being said, I'll echo some of the other comments already: The 100 is MUCH stronger (to me, even more so than an 80 with a SC - that's just my opinion), much smoother, quieter, stops better, rides better, has a better A/C (and rear A/C in most), better fuel mileage, etc.
Again, because of the ruggedness of the 80 and the way it looks, I'd really like to tell you it's a better truck than a 100, but it isn't. Like was said before, prolly the only thing that can be criticized about the 100 is the IFS, which many think isn't as good off-road, but is WORLDS better on-road!
If you're this close - and if you drive it - you'll likely buy it.