If all you want to do is watch some flash content and light internet browsing then you really can't buy a machine that will disappoint. If you're expected use is only that, buy the cheapest you can find and you'll be happy enough. $500 gets you a lot of computer these days.
The laptop/desktop split is a little muddy these days. The cheap desktops don't offer much more performance for the dollar over a laptop, and it's academic for normal web use. You'll get more harddrive space with a desktop, but does that matter to you? It comes down to how you want to use it- are you more comfortable at a desk or on the couch? I'd argue that, long term, the laptop will be a better buy. Even if you end up upgrading down the road, you can pretty easily keep another laptop running and useful. Keeping multiple towers in service gets a little difficult.
Obviously you'll want to make sure that a laptop you look at has the correct type of disk burner. I think they all come with the typical DVD ROM formats, but stuff like Blu-Ray, light-scribe and DVD-RW formats may not be as common.