Although I have nothing to say about the Libra, I'm wondering what you've heard is the downside of the rip-cord weight release. IMO it's the best integrated system on the planet, enabling you to use any type of weight that will fit in the pockets with a super easy way of putting them in and taking them out (via the zipper). You never have to worry about velcro aging or the weights falling out, unless you're using 1# bullet weights, those will slip through the threading on the rip-cord. The rip-cord is easy to rethread, and considering that you only have to do it if you've used it in an emergency, the chances you'll ever have to rethread are slim to none.
What I will say about your decision is to pick the one that fits you best. The Zena has 6 points of adjustment on the body and a 3 piece modular system designed to give you a custom fit. Once you've properly adjusted it on your body before a dive, it will never move. Yes, I'm a bit biased, I have well over 500 dives on my Zena and after 9 years it is just starting to show some wear.
All that said, pick what fits you best and what you're most comfortable with. No matter what anyone says about certain types/pieces of gear (including me), fit and comfort are your #1 priorities when choosing equipment.