I can't say really. Our room, like many, overlooked the pool, but we didn't seem to hear much noise. The beach is right there, very walkable - also maybe a good place for kids.

As far as I know, the pool is accessible only to hotel guests as a general rule.

As with other hotels, generally the difference is in location/view, size of room, amenities offered. Of course, the price goes up as all this improves. More to your question, our experience with Shutters was if there is availabilty, if you reserve a specific category of room, and when you arrive you still don't like it, you can request a different room with no increase in price. If occupancy is high though, they may offer a better room at an increase in rate. I can't recall which category we booked, but it was probably the least costly category. You can go on their website and research what each category offers.

I believe you have to specify that you want to have the twin bedded rooms. I usually call the hotel directly, and then ask them myself which room I would like to have. Last time I booked, I did it through a travel agent, which led to me getting another type of room. I recommend calling directly.