I own a panthera and I wholeheartedly recommend it. I have no experience with the obsidian, but from what I've heard it is very solid and also a good choice. They use the same parts when they come stock, but as you've pointed out the Panthera is better for modding because of its poppable top, however this comes with a caveat. The top of the panthera is less solid than sticks with unpoppable tops. Some people refer to it as a "squishy" top. The pantheras squish isn't nearly as bad as the te2, but I guarantee it's not as solid as the obsidian.

Thus this becomes about purely preference, for both you're Going to have to peel the plexi and artwork off to change the artwork. The real question is are you going to do enough modding down the line to permit having a squishier top or is it more preferable to have a solidly built tank.