Both amps sounds great. HR sounds more articulate... "sharper" maybe? MTL sounds more neutral. But it sounds mostly like a slight voicing difference pretty early in the chain. Otherwise very similar amps, power tube differences aside.
Also, the HR having the Pull Shift Treble to scoop out some mids and re-center the Treble knob to a slightly higher place goes a long way to making it sound more modern. I wonder what the MTL would sound like with the same control.
Makes me want to hear a Hell Razor KT88 vs Hell Razor EL34.
And also can't forget, that is one sharp looking alligator skin headshell. Damn.