this depends on how you look at epoxy...it's been discussed in depth many time over on rcu

most people think the longer the cure time (of the epoxy) the stronger, more durable the bond will be. the shorter the cure time results in a more brittle finished product.

strictly for fuelproofing, i would say either one would work fine. but as i stated above, mine pulls double duty (fuelproofing and firewall reinforcing) so that's why i use 30 epoxy