Two things:
1) at NYCC they have a lot of people signing but it is for a fee. The lines can be long but the talent are nice and from time to time they may walk the line themselves to say thank you for waiting - Stephen Amell did this a number of times (such a nice guy).
There are GOT cast but they not the main folks but I still go. I tend to spend a lot more at NYCC because I am paying for autographs.
2) at SDCC there is so much to do and so many people coming to sell or promote you have to pace yourself and be prepared to make a few hard choices.
There might be a glimmer of a chance NERD HQ will pull off a GOT panel. We will not know until a few days before the con or during the con itself so you may need to keep a pulse on announcements.
NYCC is actually much easier for me to get to since I live on the east coast, so I've been able to go twice. I've also had good experiences with paid autographs there, so that's a plus! I was even allowed to take photos when my sister and I met Nicholas Brendon, so I thought that was pretty cool! But, like you said, since it is paid and none of the major GOT cast members I've been dying to meet have been there, I decided against it last year.
Sounds like I will have to make some difficult decisions at SDCC, though. Though if NERD HQ did a panel, then I could probably try for the autographing and not have to worry about Hall H - sounds like it could be last minute and I may have to feel it out once I get there. Is NERD HQ offsite?