I went through the mill with this recently. 100mm or 120mm hardtail.
I got 120mm and am very happy. The fork can drop to 88mm semi lockout which is good for climbing, but 120mm is better for everything else.
I am of the opinion that 100mm is ideal for XC race. If you do more trail type riding, and maybe longer rides, the extra height on the 120mm is better, less bum-in-the-air and easier on the back and shoulders.
Also, I felt a lot of the 100mm hardtails are basically XC race bikes. Again, good if that is what you are doing.
I did run with an 80mm fork for about 6 years though. That was fine but I did have problems with sore shoulders / back on longer rides. And the difference between 80mm and 120mm is massive in terms of big shock absorption.