The ones for 10k TV are actually vr16 versions and the ones for 1k are vr15 versions of the sets when you get to max level. This is why there is a price difference
The 5th piece is useless.
Martial knowledge has spell damage x2.
Phoenix 4 piece is comparable to magnus which is craftable so it's free. Which you may as well go with as phoenix only has impenetrable if I'm not mistaken. One or two is fine for pvp, but you want some others in there. Plus you have options regarding light, medium or heavy as well as look.
Honestly, I can only see people running phoenix if they've not researched traits and they're levelling.
If you can craft or at end game, you have way better options.
It may not be ideal for V16s and/or PvPers, but I did end up getting the Phoenix set (Level 8 Superior [blue], so just 5k TV)...and, despite conflicting reports, it heals me for 14k+ health, which is full health for my Sorcerer. It's a straight-up extra life...an incomparable bonus, I think. So far it has mostly served to inform me that I underestimated an enemy and need to switch strategies.