That's tough to say - we certainly do make use of the extra space on the iPad (for example, to display the list of search results + the definition screen at the same time), but it's also a lot less portable / convenient than an iPhone. I'd say it really ought to come down to where / how you envision using Pleco - get the iPhone if you need the portability, get the iPad if you don't.
If you do go for iPad, the Air versus Mini question is something I'd suggest you investigate in an Apple Store - some people find the Mini cramped and the Air comfortable, others find the Air too big / awkward and the Mini just right.