If you're going the FB route, then strong joints are going to rely primarily on what alloys you use to braze, and getting really nicely fitted miters at the joints. Silver-based alloy is going to melt at a lower temperature and, all other things being equal, it's going to "wick" better into finer joints via capillary action, so that will likely work better for you than brass.... but it'll cost more, too, as far as materials go. Theoretically, thicker walled tubing will offer more "real estate" for your joints, but many folks have made first-rate FB frames with thin-walled tubing... All of this, of course, is based on my own research while looking into frame-building; i haven't ever brazed anything together.
As far as the rest of the frame goes and the strength of the tubing, it's again going to vary on other factors, such as the total length of each tube, intended use, etc.... but the weakest point of a frame is often to be found at the joints.
HTH
-Rob
