Which is the lesser evil?

Depends on whether you are using her base lance. With her personal weapon I'd say that +hp is better so that you improve her tanking potential.

Also even at -spd, she has a kind of decent speed stat and might not get doubled by mages and dragons too often (non-merged ofc), so the increased hp pool will kind of act as an increased resistance as well due to the fact, that she'll get hit only once.