How do these Meridians compare to each other?

Of the players listed, the Meridian 508-24 is the best sonically in my opinion. The Stereophile reviews in this instance agree with my assesment. The 508-24 has very, very low jitter which I feel gives it its ease of sound. The other players sound less defined and not as transparent. Also, another consideration between the older Meridians and the new 507 is the quality of the transport. It took a nose dive in the 507. The CDM 12.5 transport was discontinued.
I feel the Arcam is a solid notch behind the 508-20 and 2 or so behind the 508-24. It is a very smooth player but not as defined and has a digital grunge compared to the 508-24.
I started out with the FMJ then upgraded to the 508-20 and finally the 508-24. I still use the 508-24 and currently have no desire to upgrade further.

I had a 506.24 & currently use a 508.20. The 508 models have balanced outs if that's a consideration and I picked the 508.20 over the .24 because of the transport. It uses the 12.4 as opposed to the 12.5 and is a bit smoother both mechanically & sonically. I also had mine upgraded with Blackgates & Schottky's, so a comparison with a 508.24 would be skewed.

The 508.20 is a smoother sounding unit than the 506.24, although the 506.24 is a nice CDP. I picked the 508.20 to modify over the 506.24 because of the balanced outs & the transport, although if there ever was a problem with the .20, Meridian would replace it with a .24. This I know as I called them to check on a serial # & they volunteered the info.

Another consideration is that I can leave the 508.20 hooked to the system via the balanced connections & also run my headphone amp off the RCA's of the Meridian.

A review of the 508.20 http://www.vxm.com/21R.96.html

Both of the 508s are special players, but I also preferred the .20 over the .24. IMO, it was smoother sounding than the .24.

Both 508's are excellent player. I've compared 507 with 508.20 and 507 is too forward. Also the new ROM drive transport was driving me crazy. If you leave the pwr on and no disc, every once in a while it will search for disc and make this noise and you can here it across the room.

I've also compared 507 with Sim Nova, liked the Nova much better.