It's very difficult to compare anime conventions' attendance to one another, and sometimes even year to year, because there are a few different methods conventions use to determine this. Number of badges sold, for example, may count people who bought, say, a Friday badge and a Saturday badge twice. On the other hand, “warm body” counts are tough to get accurate.
Otakon issued a press release announcing that they officially predict an attendance of about 27,000, up a few hundred from last year's 26,394 paid attendees. CORRECTION: I'm not sure where Jtor got that number; I just got a newer press release with the "unofficial" final count of 26,350, a very small increase over last year.
On the last day of Anime Expo, the convention announced that they were expecting “over 45,000+ unique attendees,” though I heard a lot of others throwing around a 44,000 number from somewhere as well. Still, if the numbers are accurate, Expo saw an increase in visitors.
If you want my opinion on something to watch out for, though, it's New York Anime Festival. Pulling into the #3 spot last year with 18,399 warm bodies-- and after only three (?) years running, too. Obviously a jump into second place would be an immense leap, they'd have to grow by 6,500 attendees (about 36%), but I have a feeling it'll at least inch closer to Otakon this year. Though it may depend on whether they have more guest announcements to make!