So I've got a truly first-world dilemma: Would off season $ be better spent on a used HT 29er or a power meter? I don't have (or want) a road bike, the cross bike with slicks is just fine for my needs. So in the grand scheme of things, would a ~$1K purchase go further with a crank-based (like P2m or stages) PM or mid-level used MTB? We have great roads and MTB trails.
For me a hard tail mountain bike is just too similar to my cx bike. I know thats a stretch, but I tackle some mildly technical trails on the cx bike. If (when?) I pick up a mountain bike I think full suspension will provide enough of a differential to justify the expense. Sorry that doesn't really address your question...
What's your limiter in cross, fitness or handling?
If you got a powermeter would you change the way you ride based on the data?
I think most people will tell you a PM is the best investment for making real physical performance gains in training. That being said, in order to get the benefit from a PM, you need to be able to make a training schedule, interpret the data, and stick to a training schedule.
How do you feel about your physical performance? Do you feel like you are still making gains on your own, setting your own schedule, doing your own thing? Or do you are you wondering what you can do to make yourself better physically after last season? If you think you've gone as far as you can on your own, and you're a performance minded rider, committing to a PM and training schedule will probably be quite fruitful.
MTB is certainly more fun, and I think MTB riding and especially RACING helps you stay sharp for CX both handling and engine wise. So if you think you can still get more out of your body on your own, and aren't ready to commit to data driven training, a MTB might be a good choice. As a side note, I always wonder about people who say they are riding a CX bike on mtb trails. If you really want to ride mountain trails, you need a mountain bike.