Alcudia:
Its more spread out........with three distinct areas - being the old town and harbour - the Bellvue area and out towards Can Picafort (Playa de Muro)
It has a much bigger beach - in my opinion one of the nicest Ive seen.
It would me more family orientated
It would have less "clubs"
Although the bars and the few clubs there are - are open till the early hours of the morning, it wouldnt be as "lively" as Magaluf.
Although it has a small water park - it would be more expensive to go on trips unless you had a hire car, for example the Pirates etc.........obviously its the other side of the island.
My preference between the two would be Alcudia, Ive stayed there three times over the last 20 years........... but I much prefer Cala Millor/Can Pastilla etc so Im maybe not the right member to ask!