In this game, I tend to not heal if I get a choice between heals or tanking. In PVP, as soon as you heal, people will mark you, thus having everyone on their side make you a priority target. Not good.
Personally, I prefer to tank over healing which I then prefer to DPSing, but that's because I find DPS to be incredibly boring (using the same simple rotation as you slowly mow through targets and sometimes using a threat drop when/if you get aggro). Healing is slightly more interesting because it's not just the same constantly blasting into targets, though it vacillates between incredibly aggravating (when the tank can't hold aggro and everyone is taking damage all the time) and horribly boring (when the tank and DPS execute perfectly, interrupting everything and never standing in bad stuff) with the occasional sweet spot right in the middle (high enough incoming damage to require you to pay attention but not so much that it's impossible or annoying to keep up). Tank is always interesting because, at worst, it's just like DPS with the added complexity/interest of having to pay attention to survivability CDs (tank + spank fights) and, at best, it's about watching a lot of targets while paying attention to your survivability while not standing in bad stuff and making sure the DPS can avoid standing in bad stuff while still blowing up other stuff while beating on said targets to maintain threat while watching survivability abilities to minimize incoming damage.
Of course, if you do a lot of PvP, it's entirely different. Healing is great in an organized PvP group where you'll actually have other people peeling off the others doing their damnedest to focus fire you dead. In a random group, it's going to suck because no on will peel off of you and you'll just spend a lot of time dead and getting wailed on by the other group while your teammates just stare on. Tanking is... less than interesting... in PvP because tank survivability contributions are questionable at best compared to the reduction in damage and the only real utility you're providing is Guard (since tank capable DPS can throw out Taunt).
Inquisitor/Agents just aren't built for tanking.
Agents don't *have* a tank spec, so it's right to say that they're not built for tanking, but Inquisitors via Assassins do and it is *amazing* for tanking: great threat, great mitigation, great damage. My main is a Shadow and is always getting compliments concerning my performance so the suggestion that I'm "not built for tanking" is a bit insulting.
Which tank class you want to play depends largely upon what you want to do and how complex you want it to be.
For PvE, VG/PTs are incredibly simple to play but don't offer much in the way of growth and improvement in performance based on skill; Guardians are the hardest to learn to play and have well known problems with threat generation and damage, but turn into admirable tanks when played properly; Shadows have a middling level of skill required to learn to play them, require a *lot* of attention to fight mechanics and buffs, and have a lot of room for growth and improvement in performance as you learn more about the class and how and when to use certain abilities.
For PvP, VGs have decent control but less in the way of utility so that they're not really popular; Shadows are squishy since a large amount of player damage circumvents the def/shield/absorb they rely on to achieve tank survivability so they're generally seen running around in a hybrid tank spec with DPS gear on to capitalize on their utility, passive survivability, and damage capabilities; Guardians are highly mobile (Force Leap and Guardian Leap) and have a *lot* of utility (slows, stuns, debuffs, push) that makes them great for contributing in the non-damage media that PvP capitalizes on.
Purely in the abstract, I would have to say that healing is easier: you watch the bars, cleanse debuffs, and keep your group alive and there isn't much variation from there whereas tanking requires a lot more situational awareness and ability to control targets and plan ahead. Of course, there is a lot of variation within those roles: it's harder to play Commando heals (the hardest heal class to play effectively thanks to an unforgiving resource system and the use of multiple medium-long CDs to heal optimally) than it is to play a VG tank (watch your ammo while keeping 3 abilities on CD and spamming Hammer Shot otherwise). A lot of it depends upon your actual experience with the roles themselves: someone that has tanked in other games is going to find tanking easier because they're just learning the mechanics, not the role itself. If you're coming from neither, then healing is probably the easiest because it doesn't change much from one fight to the next. Tanking can vary greatly.