S1 is really easy and you will get it in no time. If you've done GCSE stats then it will be easier at A Level. I personally hated the Decision modules so I would be biased and I'd reccommend S2. It's a lot more straightforward than D1 or D2. I thought D1 and D2 were absolute murder and if you look at my module results, D1 and D2 clearly brought my marks down.