If you haven't guessed it, there are considerable differences between the ways helos and fixed-wing counter IMC. Helos need to be closer to the ground than fixed wing, and cannot perform their missions at all from above the clouds. "Lower (altitude), slower (reduce speed), tighter (the formation), circumnavigate (the Wx if possible), and land (if unsafe to continue)" is the helo pilot's standard weather reaction brief.
Fixed wing reactions will vary according to mission. They are more willing to take formations into the goo (no rotors to whack together) and many of their missions can be completed without reference to the ground. Think an E-2C needs good visibility to give radar information? He only needs good weather to takeoff and land. For strike aircraft, their ability to contend with weather is a function of the type of weapons they're employing and the ROE.
For tiltrotors, we've got some of the best of both. We still have the lower, slower, etc. option to some extent, but we've also got the ability to go high for awhile and come down. I'd still like a terrain-following radar and/or a multi-mode radar like the CV-22 to make it a true all-Wx platform, but it's pretty good.
I'll apologize a little for speaking for the f/w guys, but I wanted to summarize some points and I don't think I talked completely out my ass.