While I have not run those exact machines I have run a 500D, Big 40's, SA200, SAE 300, and more. I have always preferred the arc characteristics of the Lincolns. Especially for pipe.
Never ran a SAE-500 before but the 300D is nice but if you would a third option take a look at the Pipeline 200D. Thats what I would prefer the most but that's just my 2 cents.
Lincoln is coming out the a SA 300 Redface in January. Dont know a lot of details about it but it is roughly the same size as the 300D. They guys I have talked to that have ran one said its one bad ass machine.
Don't know about the red face one but the Classic 300D is back with new engines to meet EPA Tier 4i. You couldn't get the 300D for a while when the last change was happening. Could be another engine change next year ?