I think that it depends on the speed of your internet connection. There is a lot to download. I prefer the download method. You can always go back and download it again from your account and the serial number is always there in your account so you never misplace it.
I suggest getting the Plus version, as a minimum, but better to get Premium. The difference in cost is minimal compared to the cost of just Mercalli V4.
Take a look at my tutorial under the Tutorials tab about Getting started in MEP.