No deal Brexit cracks both open like a nut, but the Tories probably come out of it with more damage than Labour.
A pre-Brexit snap GE would be disastrous for Labour because it would lead to internal civil war over what Brexit stance to take. Tories as the party of Brexit are less affected.
A post-Brexit GE hurts the Tories (because stuff will have gone wrong, and blame game in full swing) but probably unites Labour, since the concern over which Brexit stance to adopt will be gone.