Although I've just begun looking at this area of development, I think it comes down to this basic difference: some tools retain the original code, and some port to native...
for instance, PhoneGap just keeps the HTML/CSS/JS code that you write, and wraps it in sufficient iOS code to qualify as an app, whereas Appcelerator delivers you an XCode project...so if you're not familiar with iOS, then that wouldn't really provide any benefit to you over PhoneGap, but if you DO know a bit, that might give you just a bit more ability to tweak the native versions after your larger coding effort.
I haven't used appcelerator myself, but worked on a project a couple weeks ago where one of our team members made an entire iPad app in about 24 hours using it.
And yes, to actually submit to apple, you'll have to get a mac, but if that's not your primary work platform you can go cheap.