Download but if its nice then i'll buy it on DVD too.
I might buy a random Anime on DVD anyway. (If its cheap) A trip to KFC costs £6 and a cheap crap film from Tesco costs around £4 or a trip to the Cinema could cost anywhere from £8-£14 so why the fuck not buy some Anime from Play/Amazon? = Same difference to me! (Order a Pizza and im alll good)
I'm usually in the habit of downloading the anime that I watch to my harddrive, though I have streamed episodes when just sampling a series (to determine if it is worth watching or not).
Truth be told, before anime was widely available online (about 15-18 years ago), I actually used to resort to buying vhs recordings of fan subbed anime. I'm so glad for how much easier the internet has made getting anime, and how great it is that video encoding technology has made it possible to preserve anime (and digital video in general) in high quality.
I normally watch it online and then download it, but seeing as my external hard drive has like 6gb left I haven't been doing that, just watching it online. If the series is good enough I will buy it.
I tend to watch anime mainly online and I tend to re-watch my favourite stuff on DVD for I've purchased it. (thats if it's out/available on DVD of course)
I like to tell myself that if it is an anime I really like I wait to see if it gets licensed in the United States and then purchase it, finding myself deleting the fansubs. So far though, that hasn't happened.
Only anime that I absolutely adore would I think of owning at this point. Clannad seasons and Aria seasons are the only ones really standing out in my mind. Plus with THORAnime releasing the Blu-Rays of Clannad After Story soon, I'm starting to wonder why I should bother buying that. >.>