Call me crazy, but I like holding the disk. That way I KNOW I have it. I use 360 anyway but if your computer takes a **** overnight then you dont have NV anymore. I would by store bought if I were you.
If our computers broke and couldnt use steam, we couldn't play the game anyway?
No reason not to go for steam. In fact, you can play sooner with Steam. It will pre-load the day before, and activate right at midnight (+ or - a few hours perhaps, depending on what time zone their activations work with) and start playing right away!
With the store bought you have to drive over, wait in line, head home, then start installing. Depending on the travel distance, whether you go to a midnight release or not, and your install time (could take an hour) you will save several hours with the Steam version.
I think they have PDFs of the manual available on steam once its out, if not sooner. I'm pretty sure they released the Fallout 3 manual before the game even came out.