I own an iPad Air2 with LTE and a Surface Pro. Here's my experience.
There's no app for Windows but the Surface Pro can run the full program in the field. I found trying to populate the form to distracting, I tried to do to much in the field, and taking photos was cumbersome. The Surface Pro is the real deal, kicks the iPad Air2 in the rear end BUT I'm collecting data in the field not trying to crunch data or complete a report.
With the TOTAL Mobile app the iPad excels at data collection; You can set pull downs for most of the fields you need to populate, checking boxes is simple, sketching is limited but simple and effective, and tapping the room in the sketch to take a photo works perfectly.
In short my Surface Pro is literally collecting dust, on the other hand I have come to rely on my iPad and use it every day. A couple of more things to consider. Just about any accessory you can imagine is available for the iPad line (hand straps, water/shock proof cases, etc.), nearly nothing is available for the Surface Pro line. As far as price, the iPad Air2 with LTE is pricey, but not as pricey as the Surface Pro line and iPads hold their value, search eBay or Amazon for used iPads and Surface Pros and there's a big difference.
Either way you have to do what's right for you. Unfortunately you'll likely waste a decent amount of money before you settle on something that you're comfortable with, I wasted roughly $1,400 on my Surface Pro experiment. But I'm happy with what I have now, I'm so happy I don't regret spending the money.
Make a commitment and don't look back, you won't regret it.
Edit: I forgot to address your time saving question. Honestly in the field it doesn't save any time, for me it's a wash at best but I would lean to a few minutes more at each property. Obviously some time is saved back at the office because syncing the sketch, photos and data takes seconds as opposed to manually entering it into the report. IMO the time saving aspect is secondary, for me it's more of a work flow and organizational tool, and the more I use it the more uses I find for it. Whether it's pulling MLS data, checking in with clients, accepting orders, etc. having an iPad with LTE has changed the way I do business.