I find I have a certain amount of cross-over too: MTB rides tend to be long enough to warrant padded lycra and chamois cream, so that's usually the good old 3/4s. Water resistant shorts over the top to try and stave off soggy bottom and holes in lycra/skin due to brambles or sudden unplanned stops. Can get a bit hot sometimes, but I found I was still taking this approach, even in the heat of the summer we've just had.
I'll sometimes wear a road top, but have learned that the braces I would usually wear on a road ride [DIY bib shorts] don't play well with a backpack. Long sleeve MTB tops are often pressed into service on commutes, tours and day rides on the road, and my shoes (Merrell style walking shoes) tend get used across contexts too.
It's probably the waterproof jacket where the main difference occurs: I'm less likely to go for high vis or expensive technical fabric when on the mountain bike. Packing down to a small size is also much more critical when everything needs to fit in the backpack.