My two cents from having all the healer and tank classes for tanking, my personal fav tank is Shadow, followed by Van and then Guardian. I struggle for w/e reason on guard tank. Healing for me is a breeze on my scoundrel followed by my sage. Commando healing is good too. I've just not played it enough to comment.
Healing is tough at higher levels b/c of the multitasking it requires. Probably one of the toughest roles to play. For me, based on mobility, defense/dodge and emergency med packs/slow release med packs, I'd role a scoundrel and focus on them as your primary healer. Also, they have one of the most satisfying pvp moves, when someone is about the die, the , which happens to give you upperhand for the quick heal and get out of dodge speed boost! Most peeps have already roled a sage. Having a sage/scoundrel healer combo is great.
Tanking on shadow at lower lvls is a bit easier d/t the number of aoe attacks, pulling from range, highest dps among tanks (used to be, not sure if that's still accurate) and none of the other tanks have the bad ass spinning kick that looks awesome when you a mob.
Hope this helps.
Obi (maker of fine 168/31 cunning barrels since Jan 2014)