Hi Kazzzza.
We have been to both in the last few years. They both have their good points. The Western Waterpark has more by way of "entertainment" and "shows" but is more suitable for older kids and adults. The Aquapark has more slides and areas for children.
This year we had to go to the Aquapark in Arenal because Western Waterpark had closed for the season. Personally, we preferred the Aquapark and are glad we went there but thats only because, I think, we have a 4 year old and there was loads for her to do.
Sarah