Personally, I don't care about the game installation because let's just say I need to leave my apartment and want to shut down my PS3 (while it's downloading), it's at 60% for an example and when I turn my PS3 back on, it's still at 60% downloading and will finish up. Once it's done, I can just install (Installing isn't really bad to me, because at least I know it's a time period I cannot turn off my console). If I cancel a download on my PS3, the demo is completely gone from my PS3 (meaning not using up any HDD space, which is limited to me due to having a 20GB).
While on the 360 (I don't really want to get into console arguments here), if I turn off my console at 60% (since I gotta go somewhere and prefer not leaving game consoles or anything on when I leave my apartment), I turn my 360 back on and the download says 42% or something, so I gotta wait longer. If I decide to cancel a download on my 360 (like at 30%), there are still contents saved on my 360, so I have to go under the 360 memory management to delete the cache. Personally, I consider that more of an inconvenience than installing demos, but that's just me.
Although, I do think there should at least be an option to auto install demos/games after you downloaded. It would be like the 360, except the 360 kind of does both, at the same time, while the PS3 will do it at the end.