If you're buying an Aqualung reg, then go ahead and get the same brand octo.
that way when you service it, you only have to deal with one shop (if you ever move, the other shop might not service Aqualung and Mares or Sherwood for example.)
The Titan LX is the "mid level" regulator. It's a good regulator, but no sense buying the Legend Octo for example for a Titan LX 2nd stage...
Might I reccomend the AL ABS Octo or the Titan/Calypso Octo. One of those two are a good match for the Titan LX.
The Calypso/Titan Octo is a more traditional 2nd stage. Just Yellow. It's also adjustable.
The ABS is a "hockey puck" 2nd stage. Some people like these, some don't. It's not adjustable, but it's much lower profile, which some people like. It also can be breathed "either side up". This doesn't seem like a big deal, but when you've got to hand it off to someone else, it remove the awkward hose positioning that you get with traditional regulators.
I've got the ABS and it works fine and I'm happy with it. but some people prefer a traditional 2nd stage also.
but either way you should be fine depending on your preference.