Need to define "tougher"....
The modern rugby league game has been watered down in terms of where you can legitimately hit someone.
On that note, it seems the NFL has a greater incidence of high impact collisions, although not as frequent as league. They also enter the field with protection (helmets and padding).
In my opinion simply taking the field knowing you don't have any protection and rely simply on the opposition playing within the rules, and knowing that high impact collisions can happen at any time, makes rugby league by far tougher.
Just the wrestle alone is tough in itself as you can get twisted against the natural movements of your body.
The contest would have to be between league and union which I probably cant split at this time