I've always found it the other way for natural; when riding stocks you can roll sooooooo much more than on mods, so it just feels a lot easier and floatier to ride? I haven't really had much of a chance to ride street on stocks 'cos I just hate the feel of them. I always seem to catch the back tyre right in the nuts on stock just 'cos I don't really have to worry about that on my mod, unless something's gone tits up :S
But yeah, stocks aren't necessarily harder to ride than mod; for a lot of natural stuff the bigger wheels roll far more easily, and for front touching you've got a greater margin for error again 'cos of the larger wheels. Seeing as you can also get super light, nice riding stocks now too there's less in the way of trouble from having to run generic MTB parts on it instead of trials specific, which has basically always been available on mod.
In a comparison of them both, I'd go mod, just 'cos they're more fun to throw about. Stocks always feel like you have to plan what you're going to do more just 'cos they aren't as flickable, but you can just basically dick around on mods :(
Mark.