I bought the elemental damage one and did some testing of that: http://www.torparse.com/a/62157
Crit percentage never got higher than 23%, so it seems the elemental/energy is force crit and internal/kinetic is for tech crit. I also did a few tests with heat signature, the kinetic one was hitting for 207 even with the 35% armour penetration from my cylinder. So as far as DPS is concerned, only Arsenal Mercs and Gunnery Commandos should get the kinetic one, all other tech user should get internal. All force users should get elemental. That sound about right?
Personally I used one on use power and one kinetic as an Arsenal Merc. On my PT I will be getting one on use power and one internal damage one, unless somebody can show something that points to other non-PvP relics.