The weather
The weather in Maspalomas and Playa del Inglés is virtually identical. Both resorts get over 300 days of sunshine per year and identical temperatures.
If anything, Maspalomas is a bit windier and sunnier than PDI because it's flatter, but there isn't much in it as the weather is both resorts is superb almost every day.
The beach
Maspalomas beach runs from the lighthouse to the dog leg where the dunes meet the sea. Then it becomes Playa del Inglés beach all the way to the Annexo II shopping centre, before merging with Playa del Cochino. Really, it's all one big beach but the two end do feel different. The central section, where the two beaches meet, is nudist.
Maspalomas beach has the lagoon and the biggest dunes so it feels wilder, and the sand is slightly yellower. The west end by the lighthouse has beachfront bars, is sheltered from the wind and is perfect for families. There's nothing to do on Maspalomas beach except sunbathe and swim.
Playa del Inglés beach is busier than Maspalomas and has a water sports centre at the eastern end. The sand is a slightly darker colour and you have a vast selection of beachfront bars and restaurants to choose from.
Both beaches have sun lounger concessions and snack bars at regular intervals.
Most people staying in PDI and Maspalomas just go the beach that is closest to their accommodation.
Things to do
Playa del Inglés is the clear winner here as it's a much busier resort than Maspalomas. PDI's shopping centres are packed with bars, restaurants and shops and most of the island's activity providers have their offices in the resort.
Maspalomas, on the other hand, is quiet and currently doesn't have a shopping centre at all (Faro II is still shuttered except for a few restaurants and the supermarket). However, it is home to the Holiday World Funfair and arcade and has the excellent Parque del Sur urban park along with lots of running and walking space.
If you love the buzz of a busy resort and your nightlife within walking distance, then PDI is your resort. If you like things quieter, then head for Maspalomas; You can always get a four euro taxi to the action up the hill.
Restaurants
There are far more restaurants in PDI than in Maspalomas, although Maspalomas is much closer to the Meloneras Boulevard and its outdoor terraces. If you like walking to a different restaurant every night, then choose PDI. Otherwise, the restaurant choice doesn't matter so much as taxis between the two areas only cost a few euros (they are said to be the cheapest in Europe).
Atmosphere
Playa del Inglés has a reputation as a party resort but this is mostly unfair. Ok, it's true that the areas right by the Yumbo and Kasbah centres are a bit noisy at night, but they are a tiny part of the resort. Most of PDI is quiet bungalow and apartment complexes with no noise at all.
If there are complaints about noise in Maspalomas, it's from people saying that there isn't enough of it. If you want to be close to the nightlife, then pick your spot in Playa del Inglés. Otherwise, most of PDI and all of Maspalomas are quiet and peaceful.
Maybe Maspalomas is a bit windier because it's flatter but there isn't much in it.
Conclusion
Choose Playa del Inglés if you like to stay close to the action; You can stay in a peaceful complex and still be within walking distance of the main nightlife.
Choose Maspalomas if you really aren't interested in going out late and just want a relaxing, and sunny, holiday.