The Apple iPads are very easy to use and are great when you have a fast Internet connection available. Apple have their own way of doing things which can take a bit of getting used to. You cannot just cut and paste between documents or move files around as you can in Windows. Generally speaking the way the Apple operating system is locked down means you are less likely to be affected by computer malware. Each application is isolated from the others, so a malware attack only gains access to the currently running app.
The Microsoft Surface Pro runs the same software and programs as desktop and laptop systems as far as the user is concerned. That makes it a lot easier to switch between devices. The downside is that the same issues that plague any Windows computer also appear on the Surface Pro. If you are used to Windows and need to swap files and data between your other computers, then go with the Surface Pro. The Surface Pro operating system does take up quite a bit of memory compared with the Apple iOS system. You will probably find quite a lot less memory available for storage once the operating system and all your programs are set up. On the (very) positive side, Microsoft Office works exactly as it does on desktop and laptop systems.
So Apple iPad if you want to browse the Internet and don't need to transfer data backwards and forwards between Windows computers. Microsoft Surface Pro if you like Windows and have other computers that you swap data to. Make sure the anti malware software on the Surface Pro is kept up to date and you shouldn't run into any issues.
For serious work, both devices will benefit from an external keyboard, so remember to factor in the extra cost. Also keep a microfibre cloth handy to clean the screen. Those fingerprints get very intrusive after a while.