So, I'll be honest - I disliked both plays.
Interestingly though, even though I liked Happy Talk slightly more, I actually think you should go with Gary. Unless you're a big fan of Susan Sarandon, and seeing her is very important to you.
The thing is, Gary is very divisive - you either love it or hate. Personally, I hated it. But many people really love it. Meanwhile Happy Talk has gotten mostly tepid word of mouth across the board. So I figure at least with Gary you have a better shot at seeing something interesting and memorable, and potentially enjoyable. Happy Talk is kind of a forgettable dramedy. Also Gary has the Tony noms, and in general it's something that more people are talking about it.
However, if checking well-known actors off your list of people to see is a priority for you, I would recommend Happy Talk. Nathan Lane, Kristine Nielsen, and Julie White are all Broadway regulars, and they're almost certainly going to be back before long, still giving excellent performances in whatever they're in. But Sarandon's last stage role was 10 years ago, and who knows when she might be back onstage in New York again?