Below are some side-by-side pics of a Small Vantage Pro and SOG Flash 1. The Pro and Avid are the same design, but they have different blade and handle materials (S30V vs. Sandvik and G10 vs. Dymondwood).
I don't have experience with Buck's implementation of Sandvik 13C26, but I do with Kershaw's. If they treat it similarly, it should take a great edge with retention being a little better than AUS-8. Cutting edge on the Vantage is 2.5" vs. 2.25" on the Flash 1. The Vantage blade is 0.1" (2.5mm) thick and 7/8" wide, vs. the Flash 1 at 0.08" (2mm) thick and 5/8" wide. Vantage is hollow-ground vs. full flat ground on the SOG. Overall length of the Vantage is 6.125" vs 5.75" on the Flash.
Hand thickness of the Vantage is 0.47" (12mm) vs. 0.35" (9mm) on the Flash. I don't have my scale handy, but the Vantage is noticeably heavier. The Vantage is not particularly heavy, but the Flash is very very light (1.2oz). The way the handle is laid out, I get a 3-finger grip on both knives. The Vantage handle is longer, but the flipper prevents gripping the top 1/2" or so of the handle. The wider handle of the Vantage does make it easier to grip though.
My Vantage is perfectly centered with no blade play at all, but there are accounts from others on the forums that were not so fortunate. Buck does carry a lifetime warranty, so if yours has problems, you can get it fixed. The Flash does have a very small amount of blade play on account of the lack of steel liners. FRN, while very tough, is not the most rigid handle material. As such, it does flex somewhat, which results in blade play on the Flash. In exchange, though, is the Flash is very light.
I like both knives, but I'd probably lean a little more in favor of the Flash 1. It really is unnoticeable when you're carrying it, and it is more than adequate for light and moderate cutting tasks. While the Vantage does feel sturdier, if I find myself needing a more solid/hefty knife (eg. for repeated heavier cutting tasks), I'd look for a larger knife (mainly a larger handle, where I can get a good 4-finger grip).