Yes, both are valid. We will not make the Gerrit template say "BUG=", because we are moving away from that footer format as we move away from Rietveld.

You'll notice that reverts similarly use "No-Try:" and "No-Presubmit",. We're getting rid of the R= and TBR= footers entirely (they'll be replaced with raw gerrit metadata instead of lines in the commit message). And the CQ understands "Cq-Include-Trybots:".

If you'd like us to change the Rietveld template to also use "Bug:", we can consider that, although I expect it won't be necessary for long.