Online and by phone pull from the same basic pool of availability. There is no difference in what is offered at either.
Online does allow you to play with dates and times a bit better, it rolls over to the next day 1 hour earlier, it allows you the opportunity to check repeatedly throughout the day or night, it doesn't close, and you can generally do it in the background of whatever else you happen to be doing. However, the web application does tend to be a bit shaky. There's a bit of downtime and browser issues do plague the application. There's also the issue with
DDP and prepaid locations not always seeing the applicable DDP and thus charging instead of holding. There's also the issue where if you're going to split up your party, the online system won't let you and you have to call.
So really, it's 6 of one and 1/2 a dozen of the other. It's typically easier for the more tech savvy to do online since we're well versed with online systems in general, while it's easier for those less savvy to do it over the phone where they can simply let the CM handle all of that new-fangled computer stuff

.
Note that certain things either have to be booked by phone (F! dinners outside of the initial offerings, shows after... Oct? I think, Dessert Party once dates are released, and possibly CP packages, we won't know that until 26 Jul) due to either program issues or just because they make you do it that way.