Be sure to check out the online help for OF iPhone to get a better idea of what you can do with it.
http://help.omnigroup.com/iphone/com...focus/v1.0/en/
And to be honest about your needs, I'll just tell it how it is:
If you want OF which is great you will pay for all three versions. The biggest thing aside from cost you will be missing is the ability to Share your tasks with others like you can in Toodledo.
I've tried everything:
Things (love it but hate that it doesn't offer cloud sync and nested objects)
FireTask (not mature enough and some odd UI conventions
The Hitlist (abandonware :^(
Nozbe (Great but too expensive)
Toodledo (can't stomach the UI)
TaskPro (what I would use with Toodledo)
Remember The Milk (much nicer than Toodledo in look and feel but still missing something)
Zenbelists (cool for simple Lists)
Egretlist (nice ToDo Evernote integration but ultimately fails since the data is locked to the device with no way to back it up or move it if you get a new device)
Awesome Note (uggggg....I have a love hate relationship with this app)
Action Method (awesome but development seems to languish)
Action Lists (great for Toodledo users and fast for entering tasks but the UI has some weird interaction flaws)
Springpadit (cool concept but haven't played with it enough)
OmniFocus (love it minus the ability to fluidly share my tasks with others)
I've tried a few others but I think that covers a good cross section of what your options are.
If you choose Toodledo be sure to get one of the other apps which sync to it well.
If you choose OmniFocus know that it will cost more but that you'll have a trusted system that works and works well. If you choose this route be sure to ask OmniGroup about their Sync Server pricing plans if you don't have MobileMe.
Hope that helps.
-policarpo | GTD2CYA