Don't see any reason not to buy blue faction stuff if you have worse gear. It only depends if you have chance to get some epics quick or if you have time for soloing.
Just be aware of what is dropping in the most popular dungeons, even the blues can be good - f.e. Cavern of Malice has rank 3 LL chest that is comparable to raid T1. If you're a cheapass then just wait for the drop
Tempest vs Juggernaut is an endless debate. My poledancer is mainly collecting dust with really crappy gear (T2/small T3, basic accessories), but she gets along quite fine with overgeared tanks in T3.