AS student here, I don't find it too taxing.
With Edexcel, I found there to be quite a bit of flexibility in what you learn. You get 2 sets of questions per paper, but out of a choice of 4. What this meant was that I could be strong on 3/4 units, and be pretty much covered.
As it happened, one unit had a terrible question (despite me feeling quite good about it) and the other unit I had completely given up on, because it was too confusing for me. I got 96% UMS in that paper on the remaining two questions.
Writing the essays isn't especially hard at AS. It's a matter of writing coherently and analytically - there isn't much room for description (unlike history, where many fall into the narrative trap) because everything you learn will be pretty relevant. You usually need to present two sides of an argument, and make a judgement at the end. That's about the extent of it - nothing you won't really pick up at GCSE.
The content can be a bit daunting if you're unfamiliar with the political sphere; as such, I would recommend you try to be up-to-date with political goings-on, even if not in much detail. It's not absolutely necessary, but it will be a huge help. Your teacher will probably help you in this respect, both of my teachers discuss current events in class, which is useful. That said, people in my class have started the course with relatively little political knowledge, and adapted sufficiently to get good grades.
I really enjoy it, and if you get opinionated about highly debatable topics, I think you will too. Comparing its difficulty to other subjects, I find English Literature and History both harder, but Sociology easier, but its all subjective.
Good luck in your decision.