Let's face it: OmniFocus induces sticker shock, and for that reason, I tried various other tentative solutions. Ultimately, I found myself embracing OmniFocus-- it's the only one with consistently flawless, transparent/automatic syncing among all the platforms mentioned by the OP. Competing apps were surely cheaper, but between their outright data loss per syncing and extreme aggravation, OmniFocus transformed, in my eyes, into the best bargain in town.
It also optionally syncs dated ToDo's to iCal, (with contexts of ToDo's flowing into the respective cals I dictate via OF's Prefs screen) and hence BusyCal ( >>> Google Calendars). It includes a no-fuss email-to-OmniFocus mechanism. I can make it as simple as desired for quick entries, yet indulge its inherent power during planning sessions. The built-in Review session is fantastic. Hence, now that I've really been using it for several months, it's like DevonThink Pro Office-- continued use brings increased appreciation for the inherent brilliance of these two apps.
I was able to dramatically kill the OmniFocus learning curve, courtesy of Google. Found a 7-day free trial to Lynda.com, which has a series of OmniFocus Essentials video tutorials. Google: lynda.com 7-day free trial
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Edit: having said that, a new alternative will soon appear: Midnight Inbox version 2. The iPad version was recently released, with the Mac & iPhone counterparts due in the near future.
http://www.midnightbeep.com/