I am looking to do a little macro work, but am not ready to invest in a macro lens. For hobby work like photographing flowers and such would it be better to invest in a set of extension tubes or in a good close up filter? I would like to utilize my Canon 40D with either the 24-70mm f/2.8L or the 50mm f/1.2L lens I already own.
I've used extension tubes in the past, with success, but what I did find was that you'll have to manual focus because the AF sensor gets really crazy with tubes. For most macro work you'll want to focus manually anyway, so use your camera's live view.
I would stay away from close up filters. They're bulky and the quality is very poor.
In general, if you don't already own a portrait lens, a macro lens with a short focal length (say, the EF-s 60 or EF 100 macro lenses) can double portrait lenses.
The two lens you own might work, but be aware that their minimum focusing distance might not fit what you need. Macro lenses are special in that they have a much shorter MFD than regular lenses - this means that you can get closer to your subject and still focus, giving you a much larger subject for the same resolution. This is important when photographing bugs and flower stamen, etc.
As for the rings / filter debate, a couple things to keep in mind:
You might also try and older toy called a focusing bellows - its similar to a ring, but its in accordion style, and lets you incrementally change your focus, allowing you to do focus stack macros. These are like the HDR of macro shots. Check out Lord V. on flickr for examples.