Neither, coming from a long time PM app user and Omni Group pro app owner.
iTaskX is a buggy POS app that needs to just die, and I wouldn't use it if they paid me. OmniPlan is a feature-gimped PM app, relative to the 3 PM apps I use regularly - 2 Windows and 1 Mac. I routinely check for better/other options, and iTaskX just can't handle real-world files as it's routinely crashing or putting out crap output with OmniProject having only a minor subset of what Merlin Project or FastTrack Schedule are capable of.
The two Mac-compatible PM apps I use are FastTrack Schedule 10.2 and Merlin Project. I use FastTrack on Windows, but the Mac app is completely feature-for-feature compatible with the Mac version. My other PM app, for which there isn't a Mac version, is Primavera. Both FastTrack and Merlin Project get regular updates.
FWIW, Omni Group is an awesome company, and their products are top-notch. IMHO the only weak link in their portfolio is OmniPlan - it's nice to look at but more competent options are available IMHO.
My GF manages several work projects in an industry that has nothing to do with mine - construction design and construction - but has plenty to do with resource management, delegation, and CRM - and her recommendation is Daylite (by marketcircle). I'm starting a new retail company in the next 90 days, independent of my existing business, and I'm hiring an attorney and my GF to manage its resources with Daylite as a back-end - I didn't tell her about my new business and evaluated Daylite on my own.