In addition to the force of the impact, a significant difference is the form that you employ in making a tackle. In football a common technique is to put your head in the center of the opponent's body in order to help stop their force. In rugby, not wearing a helmet means you are _likely_ to receive a concussion if you do this (mostly from knees). As such, in rugby a proper tackle technique involves you putting your head to the side of your opponent, and using your shoulder as the primary point of contact with the opponent.

The upshot of this is that your head is involved in significantly less direct contact (though there's still some).