Considering where I live it's summer ten months out of the year, I wear the same underwear under shorts (and long) pants all year round. I alternate between briefs and boxer-briefs with a ratio maybe 30/70 although I've been more apt to choose the former if it's leg day at the gym since I wear shorter shorts and thus don't want the bottom of the underwear to peek out.
I prefer support, security and low bulge visibility no matter what pants I'm wearing so I never go commando, although once in a great while I will choose regular boxers under whatever I'm wearing if I'll be exclusively hanging around the house that day as it still allows things to be more dangling and free while still giving piece of mind. Given that the whole point of being in the heat means more swampy areas of one's body, I'd feel too uncomfortable walking around with sweaty balls and no extra layer between them and my shorts.