6 star Deacon first.  I would build Tayrel later.  They are both useful due to weak/strong affinity.  Plus you'll get use out of both in tag arena.

Your first choice, Kael, was great, he can farm the campaign with 3 food champs and produce more lvl 60 champs fast.

If Tayrel is your 2nd and Deacon your 3rd lvl 60 or the other way round doesn't really matter. Deacon is great in arena, as he combines turnmeter boost with def debuff, Tayrel is great at Clanboss, as he combines atk and def debuff. Both have a usefull all battle aura, both can work in all dungeons. 

Yeah, Kael has been invaluable - just the difference in farming food for this champ over the first is dramatic.

I hadn't even considered tag arena too. It sounds like they'll both 6 star eventually but for now I think I'll go with Deacon since I already have the books invested in him and as a bonus any classic arena wins I can chalk up will help my great hall.

Thanks so much for the help!

it depends on your priorities

Arena? Deacon, no contest

Clanboss? lean Tayrel 

 you can use them both at Clanboss, but Tayrell is invaluable for his attack down on his A1

Dungeons? lean Tayrel

I do have tayrel and early to midgame I have been using him alongside another dedicated aoe def down champ (didnt have the books for him at the time and his longer cooldown compared to others makes him not the best aoe def down choice) you can run both, Tayrel and deacon, in same team

you cant really go wrong either way, personally I think Deacon is pure support and does well enough for the time being at 50. Tayrel kinda needs lvl 60 to perform. but both choices will serve you well

Good to hear they can both work on the same team, I'd missed the point about Tayrel's longer cd and that actually makes a difference for me. 

I just 6-starred deacon for the help in arena, and got the last book to drop his cooldowns. Now I'll grind up some food for Tayrel. I have some other options, but it seems like the two of these guys in tandem will give my teams what they're missing - especially early game.

Thanks a ton for everyone's time and input, it really helped. :)

I pulled Tayrel early, he was a massive help in Dungeons, Clan Boss and Arena. You'll be happy 6*'ing this fella.