The easiest and best way because of the disparity in the lines is to use two uniknots that will slide down the lines to meet each other. These are very strong knots and you can tie them in the dark if you have to. They also allow the line to go through the guides easily at the knot. Forget the surgeon, the nail and all the other fly knots! They are a pain.

The bouyancy thing is a VERY subtle but effective tool in the arsenal of advanced tips. For me it depends on the water. I relate to it like a flyfisher does and use the braid sort of like the floating line, adjusting my mono leader like one would for nymph fishing. Longer mono for deep and low thru big water and and shorter for boulder gardens and deep slow pools. The upright attitude imparts more wobble during a dead drift in fast current or working really deep slow pools. Another way to say it is longer tippet on boat water and shorter tippet on canyon water.