I have both, but also kept my MBP. My MBP is essentially a desktop machine now, in laptop form. It stays on my desk and I use it to handle my iTunes library, back up my own & family iOS devices, convert video for the iPads and maintain files stored on various external media, including my photos. In a few years, when I decide to upgrade my MBP, I may consider replacing it with a large screen iMac.
My 12.9" iPad Pro has then replaced my laptop needs since my MBP is now used as a desktop. I use it at my desk, on my lap with a Smart Cover and my legs up in a recliner, in bed with the Smart Cover on my chest or to the side of me and also alternate taking it and the 9.7" iPad Pro to work with me. I often run a TV show on the 12.9" while using the 9.7" in my hand for reading forums or playing games.
I use the 9.7" for all things and find it really portable, but don't feel it's a laptop replacement like I feel my 12.9" is.
I like both sizes for forum reading....but it depends on my location or what else I am doing. If I want to watch a show, then I use the 12.9 for that and the 9.7 for the forum.
I like the 9.7 for some games, like Candy Crush, but prefer the 12.9 for Clash of Clans, adventure and card games. I prefer the 12.9 for writing my holiday journal, spreadsheets, comics, magazines and regular web browsing.
I'd use my 9.7 for taking photos & video while on holiday, but I can't imagine lugging my 12.9 everywhere while on holiday, but I'd love to have it in the evenings....so my personal dilemma is about which one to take away with me if I go away for a weekend or what about a 6 week trip.
When I got my 9.7 Pro, I passed my Air 2 to my husband and then his Air to one of our daughters. She had been using my mini 2, so I now have that back as well. I thought that I'd like the mini 2 back for book reading, forum reading, some games and handbag portability, but the truth is that I haven't been using it much yet and now that I have the 12.9, the 9.7 actually feels more like a mini. Having used the larger Pro now makes the smaller Pro feel very small, light and portable. So now I think that the 9.7 Pro will become more of my handbag iPad, but sometimes I just have a preference to take the larger iPad and if I had to choose only to have one or the other then I'm not sure which one I'd pick. My head says that the 9.7 is the most portable, does everything that the larger one does and works as a camera, but the larger screen iPad on my lap in the evenings is just awesome and I don't think that I could give that up now. Perhaps if I found a new handbag to fit the 12.9" Pro in, then I'd be right.