Downloaded vs Boxed?
I suppose a lot depends on what you are really comfortable doing. If you purchase a boxed version, in most cases it will not be the most recent update. That means that as soon as you install it you'll be prompted to update. In the case of Lightroom that will require the download and installation of a complete new program anyway. Updates to Lightroom do not change the installation that you already have.
I have purchased boxed versions of Lightroom, but the only thing I have use of the box is the serial number. I have yet to ever use the disk to install Lightroom. You can always download copies from the Internet. So all you really need is your serial number.