Xander special?

It's really up to you, although most people say a 4-Turn charge won't cut it in the meta...

Bonfire is better, if you're running Fury that's 20 raw damage added to your damage output which is usually enough to waste anyone that looks at you funny.

Since I run QR, I put Bonfire because it's pretty much almost charged once Xander takes a hit. Also depends on what you have available. I've never pulled a Henry and I don't want to sac F!Robin just for Ignis

It really depends on what you need for your playstyle. If you're running quick riposte or whatever on Xander, someone can attack him, then you double them, either charging bonfire for the next round, or leaving 1 turn for ignis. Then, you can either initiate next turn and kill someone with that bonfire, or let someone else hit you again next turn and just nuke them regardless of any kind of weapon triangle advantage/defense or anything with the raw 80% boost from his crazy defense.
Alternatively, you can run Ignis with a quickened pulse seal and treat it like a super bonfire.

I use Ignis and I love it. With Quick Riposte, it makes it faster for me to use the skill. I'm usually able to use it at least once or twice in a battle, but if that's not enough for you, go Bonfire. Get that Hone Cavalry in and mmmm. Extra damage delight.