Well - I figure that if I went Macro, I'd probably want extension tubes as well,
good move. tubes on a dedicated macro lens, once you get one, will get you well over 1:1 magnification.
The 100mm and the 50mm will give you the same max size shots.
Not if it is the Canon 50mm 'compact macro.' That is a max 1:2 lens. However, the comment is true of almost all dedicated macro lenses, which are 1:1.
However, the 100mm will allow you to be farther way from the end of the lens while taking that shot. For subjects like spiders and bugs that will spook, the 100mm is the better choice by far
I agree with this. I have both a 60mm and a 100mm macro, and I almost always select the 100 for bugs. You can do bugs with a 60mm, but it's harder, and you will have fewer keepers.
because all you've done now is made sure that I won't be satisfied until I get a MP-E65
You won't get shots like the one GLJ posted with a regular macro lens, but the MP-E 65 is a highly specialized lens. You won't need it unless you are going bigger than 1.5 or 2:1. I have been shooting macro for years and still don't have one. Macro is technically very tough, and I think the best is to stick with 1:1 for a while until you are really comfortable with it, then slowly work up. A set of tubes lets you work up slowly.
I've been advised to go with around 100mm because I don't get that close to a subject to block out the light
I've never had that problem and have shot a lot with the EF-S 60mm. If you have lighting arranged right, this is not a problem.
Re your bee shots: very nice first tries! A suggestion: depth of field is very narrow at macro distances. If you can, try to keep the area you want in focus as close to parallel to the sensor as you can, to keep more in focus.
Two shots just to put magnification into perspective:
100mm macro, 36mm tube:
60mm macro, 68mm tube. (As DreDaze said, for any length of tube, you get more magnification with a shorter lens.) For scale, that is my fingernail on the left.