While this is really a contentious subject to get into and, as I read it, focused on primarily the inking capabilities of the iPad and the Surface, I'd venture a general opinion. This is based on my everyday usage of the iPad AND the Surface RT (not the Surface 2). First off, both devices are extremely well made. In terms of finish, they are excellent. But that is where the similarity (for me) ends.
There are certain tasks that are just easier on the iPad as compared to the Surface. Reading books is one example. But, to my mind, this is only because of the aspect ratio of the iPad. In portrait mode the Surface (like the Nexus 10) is ungainly. And, this where, at least in my use scenario, the so-called retina screen of the iPad shines.
OTOH, the iPad is - again in my experience - ungainly to use both as a surfing device AND as a device which one can convert very quickly into a highly mobile laptop. This is where the Surface excels. In both these use scenarios, the screen of the Surface is not a hindrance and with the Surface 2, the experience can only be better. And, interestingly, again, it is the aspect ratio - this time of the Surface - that works in its favour.
Then, of course, there is the question of apps. I think my use-case is ideally suited to compare the two. Why? Because I don't use too many apps. Though I was given a £30 credit for Apps from the Apple Store, I have not used more than £16 of it, which I used to buy book-reading apps. On the Surface, I have not paid for any apps.- and again, I don't have too many. If Office RT did not come preinstalled, then I would have bought it, but it does, and I consider that as something that enhances the value of the Surface.
How do both the devices perform? By this I mean, how smooth are they in day-to-day operation? How stable are they?
In my experience, both - aside from the initial issues involved with updating the iPad with iOS7 and the Surface with Win 8.1 RT - seem rather solid and confidence inspiring. However, I would give the edge here to the iPad over the Surface. This is because its basic apps like Mail etc. have been - again in my experience - very stable. I can't say the same for the Surface post the 8.1 update. Perhaps in a week or two, I will revise my opinion.
Lastly, in terms of battery life - a factor that gives weight and meaning to the element of mobility that each device aims to highlight - again, clearly, the iPad excels - particularly in stand-by mode. Of course, with the release of the Surface 2, and if the initial reviews are to be believed, then there has been a massive improvement in the battery life, but when compared to my 1st gen RT, the iPad clearly wins.
I would assess both devices as being similar in what they offer and I would be hardpressed to say that one is better than the other. So, what then accounts for the success of the iPad and the not-so-stellar market performance of the Surface?
Basically, I'd say, there are two core issues: (1) The iPad has the all-important mindshare by virtue of being the first consumer-grade tablet in the market. Yes, of course, I know that MS did have their tablets prior to that, but they were not really consumer grade and they were expensive; (2) The second reason follows from the first in the sense that having the mindshare, the iPad was also able to drive what I call the 'app phenomenon'. Thus, it is no surprise that the best apps are probably made for the iPad and it is this which has caught the fancy of the consumer.
According to me, there is a third reason, but that is a subsidiary one to the second reason mentioned above. I know of some very discerning individuals who are careful with their money (despite being really flush) and who have deliberately opted for the iPad. For these people, the iPad serves a clearly defined purpose and function. But a large percentage of iPad users are young and they - in a manner of speaking - have been born into the 'app phenomenon'. To them, accessing the web is via apps and not through the browser, which is why they make such a big deal out of the iTunes store and its contents. To them, any platform that does not have such 'portals' to the web (which is basically what apps are) is, in a manner of speaking, behind time. Thus, they tend to devalue such platforms. And, the more rabid sections within this population are the ones that we usually identify as the ones who have partaken of the Apple Kool-Aid!
In sum then, I think this comparative exercise - much of which is field by a media machine that thrives on sensationalism - between the iPad and the Surface (and Android devices) is an exercise in futility.