Reed's durability is innate, while Sam has to activate his power. Reed would be able to take more damage if he was attacked while unawares than Sam, but when Sam activates his power, he has the potential to rival the Juggernaut in the amount of damage that he can take. At least this is what I believe from his (Sam's) encounter with Gladiator.
A better thread would be who was more durable between; Cannonball, and Speedball.