The H are 4 layer flat copper coil. The X are 4 layer flat aluminum coil. This means that the H will have higher mass and higher motor strength. This is ideal for use in small enclosures, or in large SPL type enclosures. In a sealed enclosure your system is primarily dominated by the enclosure. However, the more mass and motor strength you have, the more efficient the system is at the lowest frequencies. In a ported system, more mass and motor means more output at the tuning frequency.
Currently we have single 4ohm goils in both the AVX and AVH. We also have dual 2ohm and dual 4ohm coils in small quantity right now.
Regarding the AV comparison to others, check out this thread on AVS. The GTG had TC2k's TC3K's, LMS, XXX, etc. All well regarded drivers. This post gives his opinions on the day. You can read further comments about the AV woofers as well.
http://www.avsforum.com/avs-vb/showthread.php?p=15938933#post15938933
John