I always prefered playing the Sentinel since it's my play style (in fact my first play through was a Sentinel), plus the sneak attack bonuses they get in combat made me feel more powerful (especially since I was all melee and only used Force Heal). Guardian always felt like easy mode to me since it's OP due to it's strengths in melee combat (plus my fastest play through was on a Guardian). Consular just made me feel week, especially considering I did it as dark side (death field only heals so much and I just couldn't use health packs fast enough, made for a lot of frustration on my part).
Light/Dark only plays a role on how you affect your crew (not as much as in did in 2 however), not on who will join you. I believe I killed that companion as well, twice and completely by accident. First time through I didn't know they could join you and second time I did in for kicks (didn't really like that companion). Some companions like you more pending on what side you play as (like Ordo likes you Dark whereas Bastila likes you Light side as examples) but that's really all it does for you.