I like the flip ups for open hill ,but they do have a quality problem. the last two butler creek obj lens covers i had both snapped themselves off with the strength of the spring with the first couple of uses. It's a bit like switches and plugs on spotlights, i cant believe that they cant be better built to last and function properly.
Bikini's probably better for close cover as the click opening and closing the flip ups can alert roe if your close in.