While both versions are "bought online," they function differently. TurboTax Online is a cloud-based service where you pay only when you file your return, whereas TurboTax Desktop is a software package you buy upfront and install on your computer. Where to Buy TurboTax Online
Turbo tax online vs downloaded software - TurboTax Support - Intuit can i buy turbotax online
The "best" way to buy depends on your specific tax needs and technical preferences. While both versions are "bought online," they function