The two-plug first-party one is best. The one-plug first-party one doesn't work with most EA games, you see, since EA had the first Genesis multitap, and their little tap (it plugs directly into the controller ports and attaches to both ports) requires, well, both ports, while Sega's slightly later design only required one port.
So, Sega games, most all other third-party games, and maybe some EA games work with the one-port Sega tap, while most EA games work only with two-port taps. A few third-party titles work with both taps. There's a list of which games work with each one. Here it is:
http://www.sega-16.com/forum/showthread.php?9857-TeamPlayer-games&p=214080&viewfull=1#post214080
On the two-port Sega tap, there's a switch. It's not labelled well, but one mode is for Sega (1-port) mode, the other for EA (2-port) mode. In Sega mode you only have to plug in one of the cords, and yes, this allows for a 5th controller, or, in fact, a second multitap. Yes, there are some Genesis sports games with 8 player support if you have two multitaps. Very few, but there are a couple. Double Dribble is the only US-released one I know of offhand, for a two-multitaps mode. See the list in the post linked above for all titles, but some are also mentioned here:
http://www.sega-16.com/forum/showthread.php?18555-Multiplayer-Games-for-more-than-4-players (Double Dribble is known as Hyper Dunk in some regions)