As a fellow C-5 driver, I would definitely recommend the C-5 route. Not only is it an easy plane to fly, but you build 4 engine turbine heavy time very quickly. Also, where it takes many years for some civilians to make it to the left seat of a four-engine heavy jet, you could be the Aircraft Commander in as little as two and a half years from the time you finish UPT. The other good part of the C-5 is the crew--some might argue this one though! Having a crew is a good deal because it teaches you leadership and it is also great to have people to hang out with when overseas.
I can't speak to the Guard and Reserve, but looking back on my career path, I would definitely have gone that route as opposed to active-duty. The reason for this is that I love flying and being the best pilot I can be--active-duty (rightfully so) focuses more on career progression and making rank that just being a pilot. Yes, my aspiration is to fly for the airlines eventually as well and stay in the Guard/Reserve as long as I possibly can. I have many friends at C-5 Guard/Reserve Units and I have not heard a bad thing about any of them.
Best of Luck!