Western Park is more modern - but more cramped
Aqualand older - but more spread out
probably the same number of chutes etc. both are owned by the same company so same prices, foods etc.
Western has shows; diving, sea lions and horses, while aqualand has larger pools
Tickets from the agencies are normally a euro cheaper than at the door, but if you pay at the door you get a €5 voucher to spend in the park. House of Katmandu also do this.
If you have children go to McDonalds in Palma Nova or Magaluf and buy a Happy Meal. You will get a free child ticket (when the adults pay).
Finally take an aqualand (or western park) leaflet with you when you go, with the leaflet you get better prices in the food outlets.
Hope this helps
Robin