Simple question, difficult to answer. But I'd like opinions. This is the first time I'm doing formal training in preparation for crit and road racing next year. I want to stick as closely as possible to the training plan, but I'm worried about the coming rainy season here in N. California. Having never owned a mountain bike, I'm trying to weigh the benefits of buying a used mountain bike for wet days. Am I putting to much on my plate trying to focus on a strict training plan and learning the ropes of trail riding. Should I just suck it up and focus on my road riding and save the money for a set of race wheels?
I'm in NorCal too. Many local trails are closed when the rainy season gets going. On the peninsula (skyline parks area) enough trails get closed that I can't make up much of a ride. Even on trails that are open you may not want to ride when they are wet because you'll cause erosion. So even when I was riding MTB a lot I gave it up in the winter.
Because of the terrain it's more difficult on a MTB to stick to a certain level of effort, unless you ride only on flat fire roads. And that's boring.
I suggest sticking to the road. Get a set of fenders, rain gear, and set aside money for replacing headsets (rain riding seems to kill mine). If you have an older bike use that as your rain bike.
Get road tires that are good in the wet. My current fave is Schwalbe Durano or Durano Plus. Conti 4000 are a little stickier, but they cut easier than I like. Chris King headsets, wheels with cartridge bearings, and Koolstop salmon pads are the deal for rain.