On May 15th, once Diablo 3 launches, I could try and get it from a local store, or I could reserve a copy of the game in advance; then bring the physical copy home and install it on my computer. I could instead buy it right from blizzard via digital copy, with the risk that I wouldn't be able to get onto the servers due to everyone else trying to download the game at the same time. What is the better choice?
You can pre-download Diablo 3 once you've purchased it online. As @dlanod mentioned, you can generally pre-download Blizzard games.
However, to more directly address the question you have proposed you need to remember/be-aware that Blizzards downloader is a Peer to Peer downloading scheme. They use small feeders to populate different sections of the game to different people and then all of those people are offered up as hosts to everyone else to get those parts.
So the situation is not really a few hundred thousand people all hitting one or two servers for the game, but a few hundred thous and people all getting a small piece pretty quickly from the server like normal web browsing or MMO traffic or whatever, and then sharing that with everyone else as long as they are downloading.
There should not really be any release day concerns with access the server to download the client to your system.
If you purchase it from Blizzard I believe you can pre-download the game client. For example I have the WoW Annual Pass and have already downloaded the game client. You will then only need to download the release patch (assuming there is one). However it is likely that this is only going to be relatively small. Overall I would suggest getting a digital copy unless you really want the artifacts of the hard copy (DVD, etc).
I find that i prefer a hard copy of a Blizzard game. They package their games in a really cool way (fold out panorama cover) and it always comes with some cool little bonus goodies.
Also, technically when you buy the 'hard copy' you also get the digital copy as well, as you can download the client for free as many times as you want. Pros of the hard copy include:
- Awesome packaging, nice to put into a collection
- Pretty much the same price as the digital copy
- Comes with guest passes, a Diablo notepad and a proper full colour manual
- Supports local retailers
- Don't have to wait ages for the game to download, it's already there on the disk.
Go for the hard copy guys, it's worth it.
When you purchase the game in box, you get the... box. You also get to avoid downloading it.
This can be good or bad depending on whether or not you intend to collect the box or throw it out.
When you buy something online off of Blizzard, that purchase is automatically tied to your account, so you can always play Diablo III. This is even a bit better than buying the game itself, since (and I know this is very unlikely) if the game is lost or stolen before you tie the CD key to your account, it won't work.
So, unless you want the box, you can buy online.
For people who are already very close to their max bandwidth, the box can be a good option!
If you pre download the client you can start as soon as the servers are up. I guess you can order it at a store but most of the stores deliver the game the day the game goes live. Means you have to wait until you receive your box (you might be lucky and you get your copy a day earlier so you are ready when the game launches) - also you could be unlucky and your game comes a day late. If you go pick up the game yourself you again have just to wait until the store's open and you have to travel there and back.
If you buy the disc. You can redeem the activation code and download it digitally on battle.net. There are no drawbacks from a 'hard' copy. Except in some cases the price is higher. Go for the hard copy if you like your retailer. Cause they have a hard time with steam and digitally downloads in general. Support them :-)