They are definitely very different games.
Diablo 2 is fun in a sort of MMORPG way -- like, it's fun to play and level up and get items and sort of 'hang out' with online friends while doing it, but if you get hardcore into it, you'll just be screaming at yourself "WHY AM I SPENDING MY LIFE THIS WAY".
By contrast, the multiplayer of WC3 is much less casual-friendly, it's pretty hard to get in to, definitely not accessible. Even if you're a strategy genius and spend a year in advance reading about the game mechanics, an experienced player will thrash you based on their micromanagement alone. Back in my heyday (patch 1.07) I was ranked top 100 solo ladder and top 10 free for all ladder, against average players I could literally take a pee break at the begining of a game, be rushed, and still win just from micromanagement. An experienced player can use an army worth 500 gold and demolish an army worth 2500 gold. It's just very very very difficult for inexperienced players to win, they don't stand much of a chance until the controls become second nature and they're at least at 100+ APM (actions per minute). Most newbs are closer to 30 APM.
So... yeah... if you plan on playing the real games, most people have more fun with D2, but IMO WC3 is a faaaar superior game. If you plan on playing single-player only, I think WC3 is a much stronger game, D2 can be pretty boring single-player. If you plan on playing mods and custom maps, I'd say they're on equal ground.
Definitely can't go wrong either way.