+res has superboon, but I prefer +hp since she need to take a hit anyway from distant counter etc. One very minor problem is that it makes it slightly more difficult to enter desperation, but with her neutral res and def it should not be a problem 99.9% of the time.
Also if you have a better blade tome blue mage (such as linde) then you could always consider giving the +hp one to them just because merging her isn't the best since it doesn't really change anything.