I think your question should be "Where those can help me most and do they realy can make a difference." And then, if someone can be used as better farmer, then good, that's a bonus.

Are you sure those are your only option? Making someone 6* is not  an easy task and sometime it may be more wise to just make some more chickens and wait for next x2 enent and then go for better champion or when you are absolutely sure you want that champ to 6*.

I don't know, maybe they are your best bet, but still, are sure they are vital to your progress? 

Overall, Fellhound is better in my opinion. You can use a ranking site like Ayumilove.com to see which are better.