Should I buy long sleeve or short sleeve onesies?

Since it's a June baby and I have no sweet clue about how to dress a baby, what do you suggest for onesies? I wanted to buy some yesterday but realized I didn't know what was best. Our summers aren't the hottest but can be quite humid. Usually between 20 and 35ish degrees Celsius. I don't want the babe to overheat but I don't want it to be cold either. If it's in a short sleeve onesie then I'd dress it with other clothes too. But if I had a long sleeve I don't know if it would be too hot. What do you think?