Also be aware the working distance -- the distance between the lens and the subject, is dependent on the FL of the lens. The smaller the FL the shorter the distance. Extension tubes only make this worse.
A 50mm lens used with extension will require the lens to be very close (a few centimeters). As you found out, that can make lighting more difficult.
My recommendation would be to eschew the zoom lenses and get a good prime lens in the 85-100+ range. I personally have never gotten a good sharp macro shot using zooms with tubes. Others may be able to do it.
Obviouisly a dedicated macro lens is the best and if *I* were spending *your* money, that's what I would get. However if *you* are spending *your* money, a good prime lens with a good set of extension tubes will keep you happy for a long time. The advantage of that would be that you would have a good short tele lens for other types of shots and you did not have to sell your child's plasma to buy a dedicated macro lens.
One other choice is to purchase a bellows and an old enlarging lens. I was able to complete my Bellows/EL lens macro set up for under $250 on Ebay and I could have gotten it cheaper if I was more patient.
Good luck with it. Macro photography is a wonderful hobby.