Last post on the matter: I have the Surface 2 and the iPad Air and I find both excellent for what I use them for.
I had bought the original Surface RT when it was first released. At that time, while I did appreciate what MS was trying to do with the designed Windows interface and the RT in particular, I found the hardware - though excellently crafted - a bit awkward. Further, in my opinion, the RT did not handle the OS efficiently and the Store was a desert. As soon as the Surface 2 was released, I picked it up (32GB with a 64GB MicroSD Card). That combined with 8.1 made it a much slicker device. Aside from using it everyday, I have taken it abroad during my travels and it has performed excellently. While at home, I have large monitors on which I view media (movies, mainly) and my partner's Nexus 10 (I also had one, but I gave it away), when travelling, I used the Surface 2 and it performed to my expectations. One point to note is that I don't have any paid app on my Surface - all apps that I do use are free. That said, I should also mention that I do not use too many apps. My situation may be a bit peculiar because (1) I don't game at all and (2) my need for legacy applications is minimal. When I am out and about either I connect to WIFI networks of which I am a member of or to those which allow free access. When neither is available, I tether the Surface to my phone (I have an unlimited data connection with free tethering, which helps).
I opted for the iPad (originally, I had the iPad 4) because of one reason only - the aspect ratio. The reason this is important for me is because I have a digital library numbering more than a couple of thousand books. Reading is a part of my job and I needed a device which I could use to that end. This is also the reason why I sold the iPad 4 and got the 32GB Air (WIFI only model) - because of the profile and the weight reduction. Luckily for me, when I bought the iPad 4, Apple had included a ?20 voucher for the Store. I bought a few apps like PDF Expert and other reading apps. I still have more than ?8 on the voucher!!! So, you see, I am not in apps - regardless of the eco-system. I find the iPad perfectly suited for reading - my library has both ePub and PDF files. The device is light and holds its charge for a long time (especially now after the recent 7.1.1 update). I rarely surf the web on the iPad, but when I do, I use Mercury browser as I don't like Safari. But aside from this, I don't use the iPad for anything else though I do have my bank's app on it (I have my bank's app on my Lumia 720 too - running 8.1 Dev Preview - and I use that). Its there just in case.
From my point of view, each device plays a very specific role and they do so brilliantly. I find it difficult to read my books on the Surface. There are two reasons for this: (1) the aspect ratio and portrait mode on the Surface just makes it unwieldy; (2) it goes without saying that the reading apps (for eg.: PDF Expert and even iBooks) are much better and more polished on the iPad. On the other hand, I have tried using a couple of add-on keyboards with the iPad, but I have not been able to get comfortable with typing long documents (and I mean book-length documents) on the iPad. I should mention, in this context, that (1) when I was trying to write on the iPad, I was using Pages. I did not like the experience and (2) the keyboards I was using was not comfortable and this was compounded by the inability to use a mouse. The Surface on the other hand is excellent - it has Office (which I use extensively), I love the keyboard (I have the purple Type Cover 2), and I can pair a BT mouse or use a USB mouse (in my experience go with the BT mouse option because the latter drains the battery of the Surface).
Some issues: I have found that surfing using IE 11 on the Surface (usually I am using the Modern UI version) drains the battery much faster than doing most other things. Second, there were some issues with the optimization of the battery at the initial stages (I am specifically referring to the Surface 2). But over time this has settled down (though some people do seem to have recurring problems with this). While I have never yet faced and SoD issues, this has also been reported to be a problem by some. By the same count, people have complained about the iPad Air's 1GB of RAM and how that is proving to be a bottleneck when using some apps, like Safari, for example. My experience, however, has been different. The Air has been rock solid and the only thing that I have noticed that has improved is battery life after the 7.1.1 update. I also notice that the interface has become a bit smoother than before but that could just be me imagining things.
At the end of the day, my mobile digital kit consists of three devices - the Surface 2, the iPad Air and my Lumia 720 - and each performs its functions very well.
Good luck with your decision!