Macro or extension tubes?

[Curly68]

I have an older version of the 100mm F2.8 macro lens but a friend is selling his 2 month old version which has the internal focusing for £280, which includes the long hood.
Is it any better than my old version with the external focusing? Or should I get some extension tubes?
Another friend has a 70-200 F4 with Kenko extension tubes and takes great shots with that set up.
I mainly take pics of my fish, so I'm thinking maybe tubes are the best way to go. I have a 70-200 F2.8 IS that I could use them on. Or should I stick to the 100mm macro?
Decisions, decisions!

[ben_r_]

With macro you should be manual focusing anyway, and I think the only advantage that the newer 100mm macro offers is the better USM AF motors which if all youre going to use the lens for is macro, you really dont need.