Low GI is the best diet for PCOS. Basically, PCOS means you're insulin resistant - like diabetics, your body doesn't process sugars correctly (or break them down), so you get sugar spikes, and the sugar is converted to glycogen fast, and makes you fat. Low Carb was extremely popular in the 20th century, but medical studies actually showed that women on higher carb diets with insulin resistance faired better (especially long-term).
Low GI foods include carbs, but mostly wholewheat ones that release sugar into your blood stream slowly. It's a real "diet for life", in my opinion, unlike Low Carb (I myself did the Cambridge diet too and whilst I lost a LOT, I also gained a fair bit back and it was unmaintainable). Avoid refined sugars like the plague!
There is an excellent book about Low GI eating for PCOS by Dr. Jennie Brand-Miller.
The Low GI Guide to Managing PCOS: Amazon.co.uk: Professor Jennie Brand Miller, Nadir Farid, Kate Marsh: Books
This way of eating is working really well for me, but it's up to you in the end! Good luck!