The downside of the RF750 was heat and tube replacements - it needs four matched octets every 2000 hours. In order to get matched octets you have to analyze and burn-in thousands of tubes. The REF250 only needs sets of three matched tubes.
That description alone would immediately discount these amps for me. Plus, you forgot to mention that as heat goes up, overall reliability goes down.
Personally, I strongly fall into the camp where the smaller amp with fewer tubes in the same model line, that is the amp of interest ( so long as the output is sufficient to drive your speakers...and room). So much so, that this is where I put my money in my recent amp addition.