Both Puerto Vallarta and Mazatlan are at about the same distance from Guadalajra: slightly over 250 miles. I have visited Mazatlan and have flown over Puerto Vallarta. By what I have seen and by what I have read, Puerto Vallarta has far more options and is far nicer. Buses from Guadalajara to these beaches take some five hours travel. I went to Mazatlan on business and after having visited Cancun, Progreso, Veracruz, and Tampico on the east and Acapulco, La Paz and Mazatlan on the west and having flown over Puerto Vallarta, and read a lot about Bahías de Huatulco, my order of preference would be: 1) Definitely Cancún. 2) Puerto Vallarta 3) Acapulco 4) La Paz 5) Veracruz, but not for its beaches (I was at Mocambo, but for its charming colonial style and heritage and its beautiful port. 6) Mazatlan.
Since I assume that you are staying at Guadalajara and want to visit some beach on the Pacific, Puerto Vallarta would be the best option. According to the distance, the bus should take not more than five hours. I saw the mountains falling into the sea, beautiful palm trees, and by what I have read in the Internet, I know that it has a beautiful water park with dolphins and other shows, and whales in the sea. Water sports are very popular here, and there is an interesting didactic tour to learn about giant turtles and see them in their habitat. Of course, the Splash is real fun for both children and adults. Click on the following link, and see some photos of the charming places you can visit at Puerto Vallarta: www.maps-of-mexico.com/photos/jalisco/puerto-vallarta/puerto-vallarta-picture-of-mexico.shtml. In the following link you will have to click on the miniature photos. Some are really beautiful. www.puertovallarta.net/espanol/galeria_photos_puerto_vallarta/photo_cd.html.
A deep sea fishing competition takes place every year during the month of November. In Puerto Vallarta you can fish sailfish and other very interesting options. Fishing tours should not be expensive here. Years ago, when the exchange rate was more favorable to the US Dollar, I rented a fishing boat in Acapulco with four sailors during half a day for fifty dollars... It may cost four times more today, but you might want to consider this option. Of course, fishing chances would be better at Los Cabos, but prices could be substantially higher. Here are some links that will show you what you can find at this beautiful resort on the Pacific: www.splashvallarta.com/index_eng.html. As I said above, the main attractions in Puerto Vallarta apart from its beautiful beach and mountains is whale-watching, playing with the dolphins, the dolphin show, sea lion show, water park, and more... While some beaches in Mexico are unique (Cancun, Bahías de Huatulco), others are very nice (Acapulco), Mazatlan is OK, but not outstanding. I have been there, enjoyed the beach, but did not find it better than any of the beaches I have been at in Argentina. And it certainly does not compare with Miami Beach. I stayed at the hotel Hacienda Mazatlan, a very nice 3-star hotel with reasonable prices and found the service very good.
Here are some photos of Mazatlan: www.maps-of-mexico.com/photos/sinaloa/mazatlan/mazatlan-picture-of-mexico.shtml www.viajesdelmundo.com/paises/fotos/151. Just across the gulf of Baja California, you find La Paz, a city founded by Spanish colonizer Hernán Cortes in 1534, where you can go on a ferry. I really enjoyed my stay there. I was charmed to see the pelicans diving down into the sea for their food. And found the area very interesting. But the best destinations I have seen in Mexico are Cancun, Puerto Vallarta and Acapulco. By what I have seen in the Internet, Bahías de Huatulco, in the State of Oaxaca, also really deserves a visit. In any case, before departing from Guadalajara, don't forget to visit San Pedro Tlaquepaque. You will discover that it is a delightful place for visiting www.galenfrysinger.com/tlaquepaque_cathedral.htm. Enjoy your stay in Western Mexico.