I've had to fight to keep my weapon a couple times (backup too far to do me any good, single car). I haven't seen the PERFECT weapon yet, but I like the features of the HK USP. External safeties are great, moderately high-cap (12+1 in 45acp), and awesomely reliable.
'Point-and-click' guns like the Glock are great in competitions, but an external safety can give you enough of an edge to survive a kissing-distance encounter. A while back, a test was done on weapon retention, and the external safety on the 1911 was most successful in providing a few extra crucial seconds for the officer to act over the revolvers and Glock-style DA semis.
As to weapon retention, I wouldn't go over a lvl II like the Safariland SLS. Remember that the harder the gun is to get out for the bad guy, the slower it is for the good guy to draw when he's in a speed crunch. Weapon retention relies more on the training of the officer than the gear.
I DO like the magazine-drop disconnect. 'shooting while reloading' doesn't hold water, 'cause no-one is going to do a tactical reload while a bad-guy is charging, and a reload shouldn't take more than a second. It CAN mean life or death in a close encounter. Dropping the magazine can be QUITE an asset when wrestling over your weapon. Drop the mag, go for the backup. Been there, did than, got the conviction. Didn't want to trade in the 4043 for a Sigma.