IMO as a DDs (Bard) I tried choco tank first, thinking that I would have free way to deal damage without getting aggro and maybe solo some boss fates but I was pretty disappointed that I got much more hate than my chocobo making it useless (I like to SPAM when I'm playing alone...). So, I turned it into attack mode and I kill mobs faster and hence get more exp... Anyway, theres always more ppl in FATES that are willing to heal you in case you get hit.