Re: Intermediate sailing class or dinghy class?
Dinghy sailing does make you a better sailor, since it's unforgiving and the feedback is immediate so you learn more, and faster. Those skills transfer easily up to bigger boats by simply getting used to slowing down the reaction factors. Think learning to drive in a stick-shift sports car versus an automatic SUV.
So for you, I'd say dinghy. But the "unforgiving/immediate feedback" factor may not be what your wife is looking for. So for her, the intermediate keelboat class might be the way to go, while you take the dinghy class. Or, just go out on that trailer-sailor JW recommended above.
Just a suggestion, lessons for her may make her feel more at ease than you "getting good enough" to accomplish the same. Do both if you can.