A little of both.
Let me put it differently. I would rather see Warren stay in the Senate except for a handful of other positions where she might be able to do more good. Among those would be NY or CA governor, and then only as a stepping stone to the White House. Since she is too old for any sort of lateral move like that (too old to run for prez, when it comes to that) I'd rather she stay put.
I do think Governor is highly overrated as far as executive experience goes. The one thing a governor, of any state, does that is relevant is they have to tight rope walk between a trillion special interests both in government and in their constituency.
The big advantage of being a Governor instead of a Senator is it enables the candidate to duck votes on really important and controversial issues and not have too many bombs in their record. That's why it has been such an effective conduit to the White House.