A lot of it depends on play style. If it were me, I'd have the druid as my main, but feral. Bear for tanking? Bah.. who needs a tank most of the time. Just leave the cat in cat form and kill stuff while the mage follows you around from a slight distance. The benefit of boomkin is that you can attack more stuff from a distance, but in the end, it probably doesn't matter much. If you go feral, you can have a bear tank whenever you need it. This way, you'll typically be able to save more mana for when you need it.

As far as holding aggro, its a little more complicated. If you are holding aggro against one character, you aren't killing fast enough. You should be in a dps form. If you are holding aggro against multiple characters, then its usually safer to kill them off one at a time anyway. The bear's roar I believe instantly puts them just a little above the person with the highest aggro, so if the mage gets a crit immediately after, it'll be a pain to turn around and try and gain aggro again. Avoid the bear in the situation, unless you try aoe'ing. If you try AoE'ing, you might as well do boomkin.