If you can cool it, get the smaller one... I am putting a cromotor on my 160lb motorcycle and thinking of kbl.. Probably the one that is $450 ... The 200/100A rated 96V version.. There aren't a lot of times when you will do 200A for more than 10-30 seconds. I wonder if 200A is a true limit or just a median for an upper range say 150-250A.
If my logic is correct, you should go for the smaller one for an ebike.
And by the way, from the way you worded it you may think that the controller limits to 80A at times. However, just to clarify, it only has one upper limit... And it is up to the user to save the controller. In other words, the smaller kbl will have more pep than the 18FET at 125A. The best way to determine what it is capable of is what and how many FETs it has... Assuming the thermal transfer is optimum
You probably are running 60-80A continuos on your 18FET with capability to do 100A for one minute (if you load up qthe motor at 125A for 30 seconds it will probably end badly). Again, it's hard to visualize the rating without modeling it