Fast weapon switching is stupid and the complete opposite of ownage.
"Oh, I missed with the flak, let me try something else! [whips out lightning near instantly] Oops, missed with the lightning, let me try flak again! [whips out flak near instantly]"
It's dumb, honestly, and only promotes switching ten times a fight and spam. You don't even have time to react to the weapon switch when it's fast.
"Hehehehe! I'll throw a rocket at him and switch near instantly to lightning to hit him when he dodges because I can switch to another weapon before the rocket even gets to him!"
With slow switching you need to be prepared for every situation beforehand because switching in mid-battle could prove disastrous. When you switch in a battle there'd better be a damn good reason. There's a reason the switch time was slowed down from UT2003->UT2004, though it's still too fast, IMO. It's just another factor in why UT200x is too hitscan friendly. Firing lightning twice in a row is much slower than firing lightning and switching to something else, and that is retarded.
Something like CPMA but altered a bit is probably ideal, kind of like a mix of instant and slow. You can't switch weapons until the one you're holding is ready to fire again, but when you do switch, it's instant. Thus, the time it takes to switch is governed by the refire rate of each weapon. Of course, there should be a lower limit to this because with things like plasma and lightning you can switch instantly since those weapons have fast rates of fire.