Extension tube (ET) versus reverse adaptor (RA):
A RA is cheaper, but you will probably have a "fixed focus" and therefore, fixed reproduction ratio -- at least on your 50mm. On some zoom lenses, the zoom ring can change the focus distance when reversed, but image quality may be poor. Reversed lenses often exhibit a notable lack of flatness of field. This may be important if you are shooting (for example) postage stamps, but a spherical focus field may actually be useful if shooting the insides of a flower, for example.
ETs are more expensive, but more versatile. You may even be able to get very expensive ones that will allow auto-focusing to work. But really, buy the cheap ones and plan to do manual focusing. Advantages: manual focus ring still works, and with your 50mm lens (at least) you can have a continuous range of reproduction ratios with three extension tubes.
Another consideration might be a bellows -- like having a continuously-variable-length extension tube. You can sometimes find these used for not much more than the cost of a set of tubes. Disadvantages: the generally don't communicate automatic aperture info, and at their thinnest setting, are still longer (smaller reproduction ratio) than the thinnest extension tube.
If you're a technical sort of person and especially if you want to do flat-field macro, go for the tubes or bellows. If you're an artsy person and saving money is really important, start off with the reverse adaptor, knowing its limitations.