In my opinion, unless you need to have two different iPads for two different locations, it's not really worth keeping both. This thread may be of use to you: https://forums.macrumors.com/threads/1518796/
I have both (along with an iPhone 5 and a MBA)
I use them for different purposes... all of the devices
Could I combine uses? Of course, but I like have a specific tool for a specific job
I use the Mini for email and as a reader primarily, as well as some IM and texting
I use the larger iPad 3 with a Logitech Ultrathin Keyboard for meetings, note taking, and for accessing my Google Drive for documents, etc.
There are numerous other uses, but these are some of the differences
Is it necessary to have both? Of course not
Do I enjoy having the best of both? Absolutely
I have both (along with an iPhone 5 and a MBA)
I use them for different purposes... all of the devices
Could I combine uses? Of course, but I like have a specific tool for a specific job
I use the Mini for email and as a reader primarily, as well as some IM and texting
I use the larger iPad 3 with a Logitech Ultrathin Keyboard for meetings, note taking, and for accessing my Google Drive for documents, etc.
There are numerous other uses, but these are some of the differences
Is it necessary to have both? Of course not
Do I enjoy having the best of both? Absolutely
I have a 3 and have toyed around the idea of getting a Mini. I wouldn't sell my 3 if I did get a Mini as I'd get hosed on the resale of it. I got the 3 just back in July and couldn't sell it for even 3/4 the price I bought it. When the Mini/4 were announced the resale of the iPad 3 went out the window.