San Diego is a great place to sail.
Did you sail out by the USC sailing piers.... or back by the Hilton Hotel?
As far as the difference between a Start board and a Go board, the 01-03 Starts came with a removable
center fin.... did the Start you used have a center fin?
The later Start boards had a retractable centerboard.... if you used this style Start ('04-'11) did you put the centerboard down?
The Go boards (all years) have no provision for a center fin, but many of them have 3 screw holes in the rails so you can attach the Starbilizer side fins to act in the same way as a centerfin or centerboard.
Where did you place the mast foot on the Start you sailed.
If you put the mast foot at all forward (from the back of the mast slot) it will be difficult if not impossible to
sail upwind with a 3.5 m2 rig.
Did you take a lesson or just figure it out on your own?
The larger 5.5 m2 rig will help (not sure how much you weigh or how big you are) but keep the mast foot all the way back in the track and use the center fin or centerboard and you will be able to sail at least 45 deg. off the true wind and maybe a bit higher.
If you have no centerfin, or if you decide to use the Go board, then you will need to stand (put your weight) slightly off center on the upwind side of the board so the rocker line tips a bit and the board will sail upwind. You must balance the upwind tendancy of the board by keeping your rig more upright (do tilt it back at all and maybe just a little forward of vertical).
Otherwise the board will turn upwind too fast and stall.
As soon as you develop some skills, there are plenty of places to sail in So. Calif. near San Diego (I used to live there) but you will need a wide board and big sails unless you go to Lake Isabella, Cabrillo Beach, or some other spots with more wind.
Mexico is good, but you really need to develop your skills before heading down there.
Hope this helps,