I depend on the turret so much, no shame saying it!
Siren is more helpful in co-op because of her healing skills under the Harmony tree. Even so, it never hurts being able to heal your own character. Good Harmony skills are Sweet Release (life orbs upon slaying a Phaselock'd enemy), Wreck (you're more powerful when an enemy is Phaselock'd), and Elated (regenerate health when an enemy is Phaselock'd). Don't get Res, as its only function is reviving a teammate. The rest of the skills of the Harmony tree are so-so. In the Motion tree, you can pick up Ward (better shields), Suspension (longer Phaselock duration), Accelerate (increased gun damage and bullet speed), and a key skill here is Converge, which will pull in enemies when you Phaselock someone. Drag them close and spray 'em with bullets or grenades!
While I don't have much experience with the Cataclysm tree, I heard that it's best for solo. I would invest in skills like Foresight, Helios, Cloud Kill, Backdraft or Chain Reaction, and you gotta pick up Reaper. It's the best skill in this tree. You can deal 40% gun damage to an enemy with 50% or above health. I would pass on Blight Phoenix (makes pretty wings, but since you're playing solo, you can never see them) and Ruin (just use elemental weapons or shock grenades for Ruin's effect). You can reach Reaper and max it out by level 25. http://www.bl2skills.com/siren.html#000000000000000000000505212500
I doubt every skill I mentioned can be attained by level 50, so I highly suggest you try out certain sets/trees and feel free to Respec your points throughout your adventure. Hope this helps.
oh wow a level cap increase? nice, i didnt know about that. i just picked the two turrets one because I love having "pets" (well if you want to call them that haha) I'm only level 15 or so though since I don't play too much, but I like the game. I think I want to level up a Siren for soloing. any tips on that class?