What are the differences between the Echo and having Hydaelyn's Blessing? [Spoilers?]

[Clawtooth]

There are likely very few or only two Echo-havers (yourself and Minfilia) in the Scions these days.

There are at least four. the WoL, Minfilia, Hoary Boulder, and Coultenet. The last two I'm pretty sure use the Echo since they're not Archons, and if they were just "hired muscle" then they'd have been transferred to the Crystal Braves by now.