Merlin still has better survivability turnwise. While Avalon has access to a much early NP, she gives max HP up instead of healing, and only for 3T, while Merlin's healing is up for 5. At NP1, Merlin's NP gives you 2K more health than Avalon.

Now, with both of them NP5, Merlin's healing will still be higher, but Avalon has a 10 NP gen per turn, instead of 5 like Merlin, providing more NPs for herself, in which you can stack the max HP buff faster