There is one major difference between Benidorm and Magaluf which hasn't been mentioned. Benidorm is a great resort with an attractive old town, good beaches and plenty of nightlife. It's also extremely crowded in the summer and this is where the difference comes into play.
There is nowhere to escape to within a reasonable distance.
From Magaluf, on the other hand, you can drive to magnificent Palma in 10 minutes, in the same time reach one of 'Europe's Top Twenty Beaches' (according to The Sunday Times), drive up the hugely scenic west coast, visit any number of wonderful towns, horse ride in the mountains, scuba-dive off deserted coves, eat in exquisite restaurants, walk in unspoilt countryside... Nowhere on the island is more than a couple of hours away and it really offers an amazing variety of scenery and activities.
There's nothing at all wrong with all day 'English' breakfasts, colourful coacktails ornamented with paper parasols, televisions permanently tuned to football or endless rows of shops selling risque t-shirts. Just that sometimes you might fancy something else and that, I believe, is where Mallorca wins hands down.