It really depends on what you plan on doing with it. If it's reading web forums while you are mobile (even just mobile in your house), the iPad maybe better. If you need more serious tools, like Word or Excel, and typing is important, I chose the Surface Pro 4. If it's to replace a laptop, which you may often use on your lap, I would recommend a light laptop.
I like to use my Surface for watching movies when I travel. I like that I can plug it in to my home network and treat it just like a desktop when I am working. It doesn't like to switch between monitors though. it always crams everything into a small box when I remove the external monitor. The screen is small and I am getting older. The pixel density is high, so seeing things can be even harder without zooming in and negating the density. Though that density does well for watching videos in HD. I like the pen too.
I personally also have an original iPad mini for reading and any iOS apps I had to have before I switched back to the iPhone. Each has it's purpose. Like seemingly every other answer on these forums, it depends what your goals are. With that, computers are my business, so we may have different requirements and comforts. Oh, and before the surface, I had a MacBook for a couple years. My main reason for switching was Office for Mac. I use Office too much for the (at the time) crippled Mac version.