The panthers are far more colorful than veileds. My panther is a decent amount bigger and WAY friendlier! I know some others have very friendly veileds, but mine hates the world and anything in it. But my panther likes to explore and paces at his cage door when I come in the room to be let out. Not all panthers are that friendly of course, I know some are aggressive also. Personally I like the color and personality of panthers but it's really a personal preference.
On average veileds get bigger. The care is almost the same except panthers like the humidity higher and the basking temps lower. It's all about preferance.
Panthers generally cost more to buy than veileds. The other thing to think about when considering buy a chameleon is the cost to care for it. They are not a cheap hobby and you have to be ready to put out the cash for a vet if needed.
Draet said it well. Be prepared ahead of time because issues will arise they almost always do at some point. For starters get your enclosure ready for whatever you decide on and have it set up for awhile so you can get the proper temps and humidity set before the cham arrives. Then get all the supplements you will need to dust feeders. Then before the cham arrives get some feeders and start gutloading so that you will have healthy food for the cham. So research whichever species you choose and get the leg work done first before the cham is purchased. Also find a reputable cham vet close to you and save his/her number because you will need it at some point. Good luck..