Generally speaking, IMO, you are better off booking over the phone. The terms are slightly different and slightly better.
That said, it's always wise to price things out exactly as you want them online - so you know what the trip is going to cost. Make certain every detail is the way you want, as any variation will alter the cost: the exact ticket features,
trip insurance, dates, number of guests. Every time you change somethign slightly, the website often reverts back to it's default settings, so it cna be a pain.
And every time you call there's a preposterously long phone tree on your dime you have to navigate, and a request to participate in a survey (remember it's all on your dime). And you have to call between 7am and 10pm EST. Also, not all agents are created equal; some even have outdated info.
But I still think it's worth it to book via a live person. Within about 24hours of booking, you should get a confirmation email (usually within an hour), and you can check that all is perfect.