Either hotel works, depending on what you're looking for. Here are a few things to consider before you make your choice.
Location: Both MGM Grand and Mandalay Bay are located at the south end of the Strip. Mandalay Bay offers a free tram that connects to Luxor and Excalibur. You can also walk inside through connectors to access Luxor and Excalibur. MGM Grand has a Las Vegas monorail station, so you can easily hop on the monorail and access the rest of the Strip.
Atmosphere: Mandalay Bay has a more casual, laidback, and upscale atmosphere. MGM Grand has a similar atmosphere, but tends to cater toward a younger crowd. Either way, both resorts offer amazing service.
Accommodations: Both resorts have very nice rooms and suites that are nicely appointed. Mandalay Bay and The HOTEL at Mandalay Bay offer a variety of rooms and suites that have a south sea theme, while MGM offers a variety of nice accommodations, ranging from regular hotel rooms and suites, to West Wing rooms, SKYLOFTS, and high roller suites at The Mansion. They also offer nice suites at The Signature at MGM Grand. Whatever way you choose to go, you will find exceptional service and a helpful staff throughout the resort.
Dining: Mandalay Bay offers a number of different dining options, ranging from fine dining (MIX, Fleur de Lys, Aureole, China Grill, Shanghai Lily, Red Square, Trattoria Del Lupo, rumjungle, Rick Moonen's RM Seafood, and STRIPSTEAK) to casual dining (Giorgio Ristorante, Border Grill, House of Blues, Burger Bar, Red, White and Blue, Verandah, Noodle Shop, the cafe, and the Bayside Buffet). MGM Grand offers similar options, with fine dining at Joel Robuchon, L'Atelier, CraftSteak, Fiamma, Nobhill, Shibuya, Seablue, and a couple other restaurants, along with casual dining at Emeril's, The Grand Wok, Studio Cafe, Rainforest Cafe, the Buffet, Wichcraft, and the deli.
Nightlife: Mandalay Bay doesn't have that much nightlife, while MGM has the Studio 54 Nightclub.
Entertainment: Cirque du Soleil "KA" You might also check their website at www.mgmgrand.com to see a listing of traveling shows that will be at their Garden Arena to see if something is there when you visit. Mandalay Bay offers Disney's The Lion King, but you might also check their website at www.mandalaybay.com to see what traveling shows might be at their Events Center.
Gaming: Both resorts offer an array of gaming to suit anyone's desires.
Other Amenities: Both resorts have nice spas and salons, along with fantastic pool areas. MGM's Grand Pool Complex is absolutely fantastic, with a number of pools, hot tubs, and a lazy river. It is hard to beat the pool area at Mandalay Bay, with a wave pool with real sand, a lazy river, and other pools. They also offer a poolside casino. Mandalay Bay offers the Shark Reef Exhibit, while MGM offers the Lion Habitat exhibit. Finally, both resorts offer a few shops, even though the shops at Mandalay Place are nicer.
Neither resort is particularly smokey, although Mandalay Bay does a better job by infusing a coconut smell in its air conditioning. Also, few people notice, but Mandalay Bay turns the sound off on all their machines, so the casino is slightly quieter than others. Finally, the weather in April should be absolutely beautiful (pretty warm, but not overly hot). As a result, it would be a perfect time to enjoy the outdoors and especially the pool. While Mandalay Bay does have a fantastic pool, the pool at MGM is also very nice, so don't make the pool your deciding factor.
Either way, you're sure to have a wonderful trip to Las Vegas.
Good luck and I hope you enjoy your trip!