Performance-wise, it's a toss up. In a blind demo with identical tuning, I doubt that anyone would be able to tell the difference between the two regs.
As
Fishpie pointed out, both are based on the same design -- diaphragm 1st stages with balanced 2nd stages.
In recent years, the Aqualung Titan LX 2nd stage was identical to the low-end Aqualung Legend 2nd stage (the one without the external breathing adjustment knob). Excellent reg. Under the guise of "innovation," Aqualung just came out with a new Legend design (MBS: Master Breathing System) which removes the external Venturi lever and integrates control of an inner Venturi "bar" into the external breathing adjustment knob. Big mistake to change a well-tested reg design, in my opinion. Thankfully, Aqualung has not implemented MBS in the Titan LX.
DiveMaven's experience with her husband's Aqualung Legend reg is unfortunate. My Aqualung Legend reg has been very dependable over the years. It breathes incredibly well -- virtually identical to my Apeks regs. FWIW, the low pressure seat on the vast majority of 2nd stages (not just the Legend) takes a set over time. Such "grooving" on the seat can be compensated for by increasing the cracking pressure a little. On the Legend, this tuning adjustment does
not require "cracking open" the 2nd stage (as
DiveMaven described). All it takes is removal of the 2nd stage from the reg hose and a tiny twist of the crown orifice with a [-]Phillips[/-] flathead screwdriver. It should take a minute or two tops. Not a big deal at all. It's a service that most reg techs would do for free for a customer who normally gets his/her regs serviced there.
One advantage of going with a Zeagle reg is that
official overhaul kits and certain reg parts can be purchased "openly" through online retailers. The same cannot be said about Aqualung reg kits/parts.
As
keyshunter mentioned, Aqualung has a
global dealer network, which might come in handy if you are on a dive trip to an exotic location and you have an issue with your Aqualung reg. There is a higher probability that the reg tech on-site will be able to fix your Aqualung reg vs. a Zeagle reg. If you go with the Zeagle and plan a far-off dive vacation, I would recommend purchasing a set of official overhaul kits and taking the kits with you.
Hope this helps...