Jelco SA-750D or Rega RB250

I have mounted both on my Sl1200mk3 with custom armboards.  Yes, the Jelco just looks and feels like a better made product.  It tracks well and works great with MC carts.  I love mine.  The RB250 is not very adjustable, mounting and aligning is more of a PITA but with right cart, it sounds fantastic, even with the stock cable. Something like a 2m blue or older Shure carts that I have.  So, it all depends on the carts you want to use really.  
Btw, replacing the Jelco mounting collar is a must.  If you tighten the set screw on the stock collar, the arm pillar will tilt and the stylus will not follow your desired alignment arc no matter what!  Easy to spot by placing a spirit level on top of the fluid reservoir cap.

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.

2channel8, did the custom collar work well?   I saw it but decided against it because it still has one setscrew only.  I guess it could have less clearance to make it work.  I got one made with 2 setscrews 45 degree apart and with a little less clearance and it worked out great.