We go to Hawaii every-other-year for vacation, and alternate between islands (Maui, Hawaii and Kauai). Among the two that you mentioned
I would take Maui over Hawaii (known as "The Big Island").
. . . more night time activities (especially in Lahaina)
. . . better shopping
. . . more shopping areas
. . . more heavily populated than Hawaii
. . . better museums (whaler, Pacific, cultural)
. . . winery and ranch tours
. . . more golf (quality in Maui and Hawaii about the same)
. . . mountain morning sunrise experience
. . . several golf-cart and/or
scooter rentals for exploring
. . . downhill mountain biking (no pedaling for two hours)
. . . many bike-rental areas
. . . longer (wide) beaches at Kaanapali
. . . good snorkel ans scuba rentals
. . . easy access to Molokai and Lanai islands (Lanai for dolphins)
. . . whale watching Dec thru Mar
There are some Hawaii advantages over Maui
. . . the Parker cattle ranch and rodeo
. . . volcano national park
. . . sloping beaches (can wade further out into the ocean)
NOTE: Our personal opinion of the main islands
Maui - Great night life and daytime activities (our favorite)
Kauai - Lush but with few activities (great for vegging-out)
Hawaii - Mainly upscale resorts
Oahu - Best surfing, but too commercial (like Miami Beach)
Lanai - Nothing but two golf resorts & good dolphin snorkeling
Molokai - Extremely rustic and basic