Weather for that week is supposed to be about the same in both locations, ~60 degrees, so it’s not like you’d be escaping a deadly cold or anything.
I’d throw in my vote for San Diego too. But I live in SD county, so there’s that bias, and I don’t really know anything about Myrtle Beach.
The city of San Diego has a lot of attractions. The famous Zoo, Seaworld, Balboa park (Which has tons of museums, as well as a nice open park), the Gaslamp (For drinking-age entertainment mostly), Seaport Village, so on and so forth. Dang. Those are a lot of things I need to do.
Legoland and the Wild Animal Park are just a drive up the I-5 and I-15 respectively.
And of course, there are beaches, but I assume that’s not a big attraction in February, and also, a place like “Myrtle Beach” probably also has beaches, so it’s a moot point.