Losing your 15 sec taunt will cripple you in your roughly 50% of the matches you would get it off.
Keeping it and leveling a second one will only let you use it 50% of the time, the rest still goes to the long taunt, meaning you will actually profit only from every fourth taunt, while it takes two times longer to get the shorter taunt to a decent level.
Losing your 15 sec taunt and just restart a short taunt will hurt you for a while, until it has reached a proper level. But the chance of interruption is smaller, meaning you can successfully execute your taunt and everytime you do taunt, it contributes to the move you intend to keep.
And a longer taunt will expose you to counterattacks even more the higher you get. The chance to render someone unable to react for a longer time will decrease, the stronger (read: more xp) your opponent is.