[Christian]

In my experience (I use OF on iPhone, iPad and Mac) it

significantly increases the use that OF has as a GTD system to own the Mac version. That is because you can enter items quicker, make tasks from mail easily, can create custom perspectives (iPhone and iPad are view-only on perspectives) and have the large screen available.

moderately increases productivity to own the iPad version since you can make use of the larger screen, better keyboard and so on.

If you do not want to buy an iPad/a mac then you will be fine with just the iPhone version. If you have the chance to get your hands on a pad/mac, go for it. If you have a limited budget but could afford either of those two (but not both), get the mac since it will get you the most added value out of OF.