Microsoft has proven that they can design a solid online offering, providing centralized friends lists, voice, chat, messages, easy matchmaking, really cool and innovative (as well as retro) games on XLA, coutnless downloads (that work in the background) and more. The PS3 forces each developer to provide their own online support or demand that gamers subscribe to a third-party service.
Online support on a console does not mean slapping on a badly integrated browser (every time I try to surf on my PSP I want to blow my brains out), it means seamless online gaming. We live in a day and age where people get connected for everything. From YouTube to MySpace, World of Warcraft to Instant Messaging, people do not want to stay alone at home, disconnected and secluded. Xbox Live brings gamers together, and that's the ace in the sleeve of the Xbox strategy, especially when paired with Live Anywhere which loops in the windows gamers too. Sony had a year to get their online act together and they have learned nothing.
I could cite many other reasons why the 360 will impress and endure, including community offerings using XNA, a non-Trojan horse HD-DVD drive, amazing non-Halo exclusives like Mass Effect, Lost Planet, Blue Dragon and Lost Odyssey, but to me, Xbox Live carries a lot of weight on its own.
Yes, I am a huge Microsoft & Xbox 360 fan (although I also own a PSP and will buy a Wii as soon as I find one), and Microsoft might not beat Sony's numbers in this generation, but one thing is for sure, both the Xbox 360 and the Wii will eat a huge chunk off Sony's 70% market share.