Easy. If all you're going to do, productivity wise, is type a document and play angry birds or whatever generic game, then the iPad pro is perfect. Be aware, it's more expensive... Surface pro 4 comes with 128 gb and the iPad pro 32 for base models. Expandable storage is a no go on the iPad, but you can get SD cards much cheaper for the surface pro, 128gb extra was 30 euros on amazon.
Ports like usb thumb drives, printers, scanners all work with Windows since it's a full PC. You can't use anything on an iPad that isn't a Bluetooth keyboard sadly.
If you want a computer, surface pro 4. If you want a tablet then any Android or iPad will work. I like to truly multitask, so the surface was the only brand name option for me.