My 50 is a Jedi Sentinel (the DPS class of Knight), so as for companions I can tell you this much: you don't get your healer till the mid-30's. Before that you'll be using Kira (your DPS) as the first companion, T7, is a tank and honestly really useless.
The Knight storyline is solid, to me it feels like what KotOR 3 would have been. Your healer companion, Doc, is hilarious and if you keep him in good gear he will save your ass more times than you can keep track of.
I've only played a Shadow to 24, and honestly I found the story a bit dry. I've heard it gets better after the first Chapter, but I haven't bothered trying it yet. Of what I've played so far, the Knight, Juggernaut and Smuggler stories have been good. and I hear the Sith Inquisitor (Assassin) story isn't bad either.
Also, Biochem is currently the best craft skill thanks to the reusable items and some stims (the equivalent of flasks from WoW).
However, in the next patch they're going to allow the crafting of Orange gear (the kind you can replace mods in to keep the look you like as opposed to whatever stuff drops for you) from Reverse Engineering (breaking down things you make for parts in an effort to learn stronger versions of the items), which means you can just buy a stack of medpacs and save yourself the trouble of a prof you're not interested in. Reusable medpacs are nice for sure, but that also means taking time out of leveling/enjoying the story to farm mats, as you'll level into buyable medpacs faster than ones you can make. Plus, vendor pacs are stupid cheap and money flows like water once you get off your first planet.
Added at: 17:26
Go Jedi Guardian. I have a sith juggernaut (same mechanics as guardian) and it felt like the iconic Jedi from the movies. The Jedi warrior story (though i played as a sentinel) is also much better than the conselor, and was again much more iconic.
Is the Jedi broken for tanking? Yeah, a little, but it won't stop you from tanking flashpoints, and end game tanking should be fixed eventually.
But it you want the iconic star wars look and story, guardian all the way.
Fyi: I built my juggernaut for dps, and enjoyed that most. Ymmv. I still tanked flashpoints, though granted i was teamed with an awesome healer.
Also, to add here. My GL is a Guardian, and at least with Normal 50 flashpoints he's tanked just fine, in both DPS spec and tank spec. I've not done a Heroic with him but I'm willing to be he does fine. We haven't done an Operation yet, but I've heard there's plenty of Guardians and the like doing hybrid specs to allow some DPS flexibility but also allow offtanking, as the number of needed tanks in Op boss fights varies so much.