As I write this, the Linksys WRT54GC currently can be purchased online for as little as $60 and the ASUS WL-530g for as little as $73. That's only a $10 premium over the Linksys' WRT54G bigger brother and about the same as you can get an ASUS WL-300g for. So why would you buy one of these mini routers? I think the answer mainly depends on how cool you think it is to have a tiny cable/DSL router.
Now they do look cute, don't get me wrong. But the reality is that these things are so tiny and light that when you plug a bunch of network cables into them, they can actually be suspended in midair by the stiffness of the cables alone. To get one to sit up straight, you may have to use bricks, screws, contact cement, or some creative twisting and bending. The point is that I don't really see the point of making these things quite so small.
To be perfectly honest, I think I'd actually prefer something a little bigger than standard if it could have a lot of pretty flashing lights on it, or maybe some kind of graphic display showing how hard it's working for me. But on the other hand, showing up at a LAN party and whipping a wireless router out of your shirt pocket like James Bond would be kind of cool.
Both the Linksys WRT54GC and the ASUS WL-530g provide a lot of functionality in a small package that won't break your budget. It's really amazing how many features can be crammed into such a tiny space. For me though, the Linksys wins out. It has a better interface, slicker design, better performance, lower pricing and is more widely available. The Linksys WRT54GC is simply a better designed product. Unfortunately, the ASUS WL-530g by comparison just doesn't seem quite as well thought out.