A little more details with comparing the two cons.
NYCC
Smaller convention center so smaller floor space. What NYCC beats SDCC, imo is the artist alley. If you like/want to meet comic book artists, NYCC is the one to go to. Much more artist head to NYCC from what I've seen.
Tv and films panels, just doesn't have much compare to SDCC. Maybe its because of the distance of LA with all the production offices or maybe the airing season of the hall shows started already? Idk. Sure you may have SHIELD, Gotham and Walking Dead this year, but thats really all? Compare the number of shows to SDCC, SDCC blows NYCC out of the water with tv and films.
However, I feel like more actors/actresses coming to NYCC to do signings/autographs. Esp the ones who do the convention circuit. Maybe its not the number of them, more like the accessibility of getting their autographs/photos because NYCC has a designated space just for that.
Because of the location of the Javits Convention Center, its out to the edge of NYC, a b*tch to walk to really. This year they've started to do more "off site" events, I haven't gone to any but thats the NYC problems. Its too spread out. Its very easy to get distracted by everything else NYC has got to offer.
SDCC
I like to refer to SDCC as THE CON because it is. First year I went, I was completely overwhelmed. So much happening so much to do. One of the off site events the first year I went, was Flynn's Arcade from Tron. I didn't even find out about this till the con was over. The SD convention center I'd say is about 30-40% larger than NY, floor space wise. Much bigger rooms too. Hall H holds over 6000 and Ballroom 20 holds 4000, compare to the largest room in the Javits was about 3000 I believe. I was one of the lucky few who go to see both Avengers previews.
Exhibitors wise, all the big dogs come out here. WB, Legendary, Fox, Marvel, DC, Sideshow, Hasbro, Mattel and so much more. Already thats so much to see/do/wait on line for. Much more tv and films come to SDCC to advertise. Not all will offer autograph signings but if they do, thats another line to wait on to get wristbands to get autographs.
This is what I believe SDCC beats out NYCC, its the location and its surrounding area. All the major downtown hotels are across the street from the con. Literally walk out of the hotel, cross the st and you're outside the convention center. Restaurants and bar are well within walking distance if not, downstairs from you if you're able to get a downtown hotel room.
Because of this convenience, SDCC takes over all of downtown San Diego. For the last couple of years, Walking Dead had its obstacle course at Petco Park, baseball stadium across the st from the con. Game of Thrones took over a gallery and had its event across from the con or right downstairs from my hotel. Couple years ago, WB brought all its batmobiles out and put them out on the lawn on the Hilton, across from the con. Assassin Creed had its obstacle across outside a hotel and last year rented out a boat and put it in the back of the convention. Theres just so much going on and just can't do them all.
Oh and most celebrities stay in the same hotels as you (downtown hotels), so its very likely you can run into them in the elevator. I rode the elevator with Kevin Bacon once, my friends met Eljiah Wood in the lobby. I ran the Walking Dead Escape with the black power ranger this summer.
I know the argument is SDCC is way more Hollywood than comics now and its true. SDCC has become more of a pop culture convention than comic convention. I don't see that as a bad thing at all. Breaking Bad has nothing to do with comics but they came last year and everyone loved it.
Did I mention the weather in San Diego? Man escaping to SD in July from NYC's humid summer is the best thing to do. SD has some beautiful weather although for a couple days this year, it was kinda hot.
Thats my comparison of the two cons. I know it sounds like I'm talking crap about NYCC and I am. You want to go to the best con, SDCC is the way to go. I love NYCC because its the first con I've ever gone to but once you've experience the comic high of SDCC, it becomes your standard and you'll wait more.
Side note, NYCC is produced by Reedpop who also produces Book Con, Star Wars Celebration, C2E2, etc., a for-profit company. Which is way you'll see those VIP passes.
SDCC is produced by Comic Con International, a non-profit company that also produces WonderCon, SDCC's sister show in the spring. For 1 price, you'll get a chance to do everything in SDCC if you can wait on the lines of course.
@mllNY might want to chime in here as well, as he has gone to both.
Can't wait! And at the end of the month, in Downtown PHX there's gonna be the annual ZOMBIE WALK!!! 
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