I am using Merlin since version 1 and I still use it for complex projects. It is a very powerfull application. In other words, if you are new to Merlin then you have a steep learning curve ahead. I started playing with OmniPlan only recently and I like the user interface. In many cases I find OmniPlan more intuitve to use. I think Merlins user interface needs improvement. On the other hand, Merlin is more feature-rich and more mature.
YMMV
YMMV