In my opinion, the biggest advantage of cutting your own guides is that you fine-tune them to your table saw's miter tracks. The miter tracks have manufacturing tolerances, and are probably not exactly 3/4 inches. Worse, each miter track is probably a little different in width.
When cutting your own guides you can sneak up on the width for each track to have zero play while moving smoothly in the track. This should give you a guide rail that is the perfect width throughout its length, providing full support to avoid side-to-side movement for the entire cut.
The main disadvantage of wood that it will change dimensions with changes in humidity. Choosing a type of wood that has minimal variation with humidity changes (such as cherry) as well as aligning the growth rings to run perpendicular to the face of the table should minimize the problem. Plywood would be more stable, and some people use various plastics (HDPE springs to mind). I have had great luck with Baltic birch plywood for my guides.
Metal guides are much harder to fine tune, both if they are too narrow and when they are too wide. In addition, you are combining manufacturing tolerances on both the miter tracks as well as the metal guides. To counter that, most commercial guides have little plastic cams or small screws that you can rotate until the fit is perfect. But this only gives you so many contact points, and I would be concerned about their long term durability (since there is a small contact area rubbing against the miter slots).
Finally, the plan to move the rails from jig to jig will probably not work in the long run. Using small wood screws to hold the jig to the rail won't hold to many assembly-disassembly cycles. Using machine screws with nuts would probably be better in that regard, but I would still be concerned with re-alignment issues after reassembly.