Having been almost every year for the past few years and stayed onsite, I would say it is invaluable to stay at Hotel PortAventura/Villa Mediterranea or Hotel Gold River as they have direct park access, so you can just walk in and out as you please, plus they tend to let you in to the park earlier. I'm not sure how they price up to the others though. Generally the best rates are booked for offer periods through the official website.
You can dine-around at any other restaurant on property with the half board plan, so you can still explore Hotel Caribe if you stayed at Hotel Gold River for example. Also you no longer get complimentary Hotel Express access, but there are heavily discounted prices available for hotel guests.
Are you thirsty?