I would definitely suggest reading through this thread (My First Router, Built in Steel) by "Haydn" before you get too far in. There is also one by "oneyaker (New home made MadVac CNC router)". These two are probably the best steel router builds that I have seen.
I think that part of the decision whether to build using steel or aluminum extrusion depends on your plans for using the router. I would think that the steel built router could end up being more accurate than the aluminum router, but if you only need +/- 0.010 either is going to be fine. If you are trying to build to achieve +/-0.0005 repeatability like "oneyaker" then I would say steel is probably the better solution.
There is also the question of what you are planning to cut. If you think you might occasionally cut aluminum but mostly wood or mdf type things, aluminum would probably serve fine. If you think you will cut mostly aluminum and occasionally steel, then I would definitely be looking at steel construction (and possibly a mill instead of a router).
I started a welded steel construction router and I wish that I had gone for bolt together construction instead. I got the table built to fairly precise standards but I stopped and built a mdf router to learn more about the whole cnc process before finishing the steel construction router. When I finish the reconstruction of my current mdf router, I may actually cut the steel router table apart and rebuild it using techniques like Haydn used.