Mike, I have used the AR400 in zagis, gliders,etc . My MX2's and the like are using AR600 (full range) used extensively with no problems. Albeit, for a really locked on RX, I would use the AR6210 which comes with a satellite. The latter having effectively 4 antennas.
Really down to what you want spend and feel comfortable with. Nearly all my models except the big stuff use AR6210 (DSM & DSMX) which is the updated version of the AR6200. All of these are compatible with a DX6.