Hard to really do much comparison between the two places. I found them to be so much different, yet enjoyable. People are good in both places, but have different agendas, different life styles. One has the Lake, the other, the Jardin and food, etc. One not important to many, nor the other to others. Kinda like Gto., so different, but with many pluses for those who live there. All have their problems, but somewhat different. All worth a trip and several days to enjoy.
I lived in Ajijic for 18 yrs and moved to SMA nearly a year ago. I find the weather about the same but the nites are always cool. We´re having overcast days now and I find that if you do everything you need to do before 1 or 2, then go out again at 6, you avoid the hottest times. I miss the dissipating lake, but have seen it come and go thruout the years, I love the sophistication of a larger city, there are MANY, MANY things to do. I actually have an active social nite life here, whereas in Ajijic, I found it difficult to go out at nite and drive 12 west. They´re both great places, depends on what you´re looking for