Jelco SA-750D or Rega RB250

Don’t be too worried about the Jelco collar comment. While I agree it is a good idea, it is not necessary. I run a Decca cartridge in mine and there are no tracking issues.

noromance is right! I've been using the 750 for a few years and never changed out the collar and yes I've known about the tweak for a long time. 

But which tonearm sounds more welly? Which one has more midbass bloom?

It doesn't work that way. I mean if you stick an AT440MLA on either tonearm, you are not going to get midbass bloom. 

As for the collar, yes the stock one does not cause severe problems and is fine for most people.  However, if you try to print out and use an arc protractor, you cannot trace the arc well because the arm pillar is not vertical.  May or may not be audible to you but what I say can be easily verified.

So you're telling me AT440MLA will have the same exact midbass on both arms? I find that hard to believe.