Longer lenses result in longer working distances, which often makes life easier.
Also many extension tubes state that they should be used with lenses with a 50mm or longer FL.
Closeup lenses:
Pros - You maintain AF, and are guaranteed to retain automatic aperture. In short, maximum compatibility and ease of use.
Cons - Very few closeup lenses are achromatic doublets. Those that are will be quite expensive. Non-achromat closeup lenses will add chromatic aberration. I've gotten decent results with my +1 and +2 Tiffens, I haven't tried the +4 diopter lens on its own, but the results with all three stacked were abysmal - severe CA and general blurriness. The set was $50.
Extension tubes:
Pros - No degradation of image quality
Cons - With a few very rare exceptions, you will lose AF. Although in the extreme macro range, unless you have a lens with internal focusing (neither the 18-55 nor the 50-200 do) AF will not work as the focusing mechanism will change the working distance. You'll need to move the camera closer/farther away from the subject to focus. (this is why people sell focusing rails, but if you have a short tripod with an extendable center column this will work OK for straight-down macro shots.)
Also, cheap extension tubes will remove auto aperture functionality, which will stop any P-TTL flashes from functioning and make exposure a general pain. More expensive ones will preserve auto aperture.
In short, the extension tubes will be more difficult to use but will provide better results.