I have an iPhone 6S+, iPad Pro 9.7, & Surface Pro 3.
I use them all, but my iPad has pretty much become my main computer. There's certain heavy duty work that I must pull out my Surface for. Plus, one of my printers neither the iPhone, nor iPad will connect to, but the other printer it will connect & works perfectly. That being said, sometimes that's the only reason I fire up my Surface, so that I can print something with that specific printer. I use the Apple Pencil all of the time.
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I love using iOS, you can tell it is designed "touch first" MS Office is pretty powerful on my iPad, usually my needs are taken care of. The battery life is amazing & I love how I can pretty stop using it in the middle of something, throw it on the sofa...Then come right back to it later & it's ready run perfectly.
I would definitely recommend the iPad Pro. I'm confident Apple will support it, the hardware is good, & it will retain it's value. There's even things I used to do on my iPhone that now I do on the iPad.
So, yes , I use my iPad as my main computer, visiting my Surface Pro 3 maybe once a week or less...With the exception of printing which is maybe 5 minutes of work. Fix that printer issue & I would use my Surface even less!! There's no question of which is more powerful, but I don't need that extra power all the time anymore.
Unfortunately, I still have to do my back ups on my PC. It does make my Surface almost feel like a brand new machine when I do use it because it's not very often now.
**I no longer have to visit my Surface to print regular documents, the iPad (and iPhone) are now able to print to both of my printers**