While both options are good, it would be best to see the plants in person. You must get to know your choices personally so you can see the plants' authentic colors. Buying the plant personally means that you can also check on its physical state and see if the actual plant is healthy before you buy it.
It would be okay to buy plants online for more expert plant people if it's from a trusted plant store. It is also good if you know some feedback regarding a specific store's online service so that you can opt for their online options.
Either way, you should  be vigilant when buying plants to ensure that you get the most out of your purchase.