I've lots the airspeed indicator in IMC twice now and didn't find it
effected the flight awhole lot. Both times were during climb out into
low IMC (stuck spring cover). However, if I had to pick, I'd probably
take the VSI. Nowadays with ok GPS altitude you could also lose the
alitimeter and make a reasonable attempt with the GPS altitude after
figuring out the difference from ATC's mode C response.