Focusing rail or Macro extension bellows

I am still not sure whether to get a focusing rail or macro extension bellows. What are the pros and cons? Thanks.

A bellows and a rail do two different things: a bellows gives you essentially a variable extension tube for variable magnification. A focusing rail allows you to move your entire rig around to get things framed and focused the way you want. IMO the rail is far more useful, though I have both, and occasionally use the bellows as well.

You can use the Bellows to focus, but usually they are not intended to be used for that purpose. The macro rails I have move much smoother than the bellows I have. I wouldn't try to use a bellows as a substitute for a rail unless you've been able to try the exact model in advance.