No reason Congress can't do both. The impression (and reality) is that they are consumed with the investigation. I have the good (or bad) fortune to have had direct conversations with members of Congress and stuff they said to expect to happen (lay-ups) isn't even close.
I don't disagree that they are consumed by impeachment currently. There was smoke to be perused regarding obstruction of justice, but the Ukrainian issue lit a fire that can't be ignored. It's not like congress has been effective leading up to it though. This is a consuming topic and it will distract congress from being ineffective to slightly more ineffective.
Politically for democrats I don't like impeachment. In principle though I think there is more than enough to warrant them moving forward with the hearings.