To try to answer your question as how I see it (size comparisons):
I have an iPad (the NEW iPad)
I use it for surfing, playing Zynga Poker before lights out, and streaming any video from my iMac to my ATV downstairs. If it weren't for the need to stream and facetime with my folks) I would likely get rid of it for something more modest, but I have it so why replace it.
I have the S3 and I LOVE the screen size. It makes any sort of web surfing vs my old iP4 MUCH better. However, that being said, I would not like to have to do any considerable or lengthy research or website scanning on the phone because a tablet just has much more screen real estate for that kind of information processing.
If I had to choose between a tablet OR a S3. Quite simply I would choose the S3 because overall it is just a beast and works as a phone, mobile gaming machine, newsreader, (I am also now attempting to see how I feel about it as an e-reader but not certain as I love physical books), scanner, etc etc. It is a great phone, great to text with, average photos from the camera (nothing to write home about). It is also a decent GPS if I need it. Basically if I had to choose, i woudl choose portability over size.
The form factor may be somewhat close 4.65" vs 7", but I would still choose a larger screen for the tasks that I mentioned I use my iPad for.