That's a good question. I don't know if he's fallen out of favor or what, but he really flashed during the pre-season. Quick penetrator with a high motor, he looked like one of those guys that was going to make everyone else on that line better, but he's pretty much vanished.
0.5 a sack, 4 tackles and while raw sack numbers don't tell the whole story of how much pressure a team is getting, the opposing quarterbacks always look a little too settled in the pocket to me when playing against the Bengals. Marvin favors a more read and react, two-gap scheme where the linemen funnel the action to the linebackers instead of focusing on getting up field wreak havoc then pin their ears back on 3rd down. I'm not quite sure Zimmer has completely done away with that style, but I think Geno would be more effective in a one gap.
Depends on how you measure your corners and what you expect from them. No question Joseph is a better athlete, better speed, better hips and better man-to-man. That alone is why I give a slight edge to Joseph, because if I've got a guy who I can stick on an island it frees me up to do so many other things scheme wise.
Hall is a more complete corner. He's not a true one-on-one, shutdown guy, but he is physical, good tackler, has great ball skills and is pro bowl caliber when he can keep his eyes in the backfield and have a little help over the top. That's why yesterday's game against White doesn't shock me that much.
Hall is a really good player but he doesn't have great hips and that's what I look for in a corner because you'll be susceptible to double-moves. Last year's game against the Chargers where we had the late lead and Vincent Jackson just manhandled him and had him twisted all around comes to mind.
This vid is a perfect example:
http://www.nfl.com/videos/nfl-videos/09000d5d81525696
When it's all said and done, though, their skills are the perfect compliment to one another. They are better as a whole.