I am also one that has both the retina iPad mini 2 and I recently just upgraded my iPad 4 to the iPad Air 2. Both iPads are the 128gb model with LTE. I enjoy having both iPads and I do use both of them more than I thought I would. I often have both of them with me. I know, seems a bit much to carry 2 iPads and an iPhone as well.
I will often use the two iPads at work while on break, the mini I will play a video or movie and use the larger iPad for checking emails, Facebook etc. or even working on some plans and excel or numbers spreadsheets. At home, often the same thing. With iCloud, One Drive and DropBox, about 95% of my things are in sync with each other so it doesn't matter which device I work with.
Sometime's I will just take one of the devices with me and that will be fine, because I know that things will sync to each other and the iMac at home. So for me, having both the regular size iPad and the Mini is yes. I will admit though that I do use the larger iPad more than the mini, but the mini is also used more for watching movies and stuff while working on the larger unit.