If you don't see a lot of use for it, I would certainly skip the hydraulic auger. 
Just go with a used 3 point off Craigslist (I see them all the time) and use it for your pole barn (because yes, you NEED an implement 

) and when you are all done, go put it in the weeds for a couple years until you need it again. 
IF, you were going to go hydraulic, I recommend putting it on the backhoe if possible. Seems like it would be a very slick setup. 
I do not like the FEL mount. It doesn't have as good Vis, it's a pain in the rear to hook up, and you lose the use of your bucket or grapple. At least the 3 point or backhoe mount would enable you to do something (anything) with your FEL. 
In addition, a lot of 3rd function valves for the FEL are run with 1/4" hose. If this is what you have, it's not big enough to get full flow to the auger motor. 
Then you need ridiculously long and cumbersome hoses. 
I don't see what all the fuss is with the FEL mount. Yes, you get down pressure, but they make those for 3 pt mounts as well.  They just aren't as "great" as you'd think by what you read online, in my opinion. 
I think JD even has down pressure on the 3 pt hitch. I can't believe more companies don't do that!