There are pros and cons to both. If you get a baby you will get to see him grow for his whole life. The con to that is while they are usually very strong the adults are always going to be stronger. But if you do your research you shouldn't have a problem at all.
Pro to buying an adult. Like I said they are going to be very sturdy. The con is you didn't get to see him grow up and he's going to automatically going to have a shorter life.
My opinion. If you are buying it for a pet get a baby. Do your research and ask all the questions you think that need to be answered.
This is by far the friendliest most helpful forum you will find on chameleons.
**jacksons can be very tricky as babies. So if you get one make sure it's atleast around 4 months.
Veileds really aren't that tricky at all but make sure he's atleast 3 months old.
Make sure you buy captive bred. Wild caught animals can be problematic sometimes.