29er vs hybrid/cross bikes???

I'm confused- what's the difference between a 29er MTB and say a hybrid or cross bike with wider tires? Just the frame? What's the benefit of a 29er over an upright-ish hybrid/cross frame?

A 29er mountain bike should be sturdy enough and the geometry should be set up so that it is a good all round mountain bike, i.e. can survive on really technical mountain bike terrain. A cross-bike is just that, designed to ride on pavement and dirt; but unless you are a really, really good bike handler, they do not do that well in the real technical stuff. They have narrower tires (cause they don't have the width in the stays to take a really wide tire like a mountain bike or 29er mountain bike) and since they are narrower tires, you will get flats easier on a cross bike in technical rocky riding much quicker than a 29er mountain bike; and a hybrid is usually more like a mountain bike than a road bike, but is not really manufactured for the really technical off road riding of mountain bike technical terrain either.

So by technical MTBing you mean climbing wooded and/or boulder-filled areas? If I was looking for something to do some non-paved (rail trail, dirt roads/paths) biking but still have some speed I'd want to be more hybrid or cross level, right? Do the 29ers weigh more since they can handle more?

A cross bike or a hybrid would work for unpaved paths not bouldery rough areas. Some cross bikes can take wider tires than others so you may want to look into a wider tire if you want the bike to be more stable. Weight on a bike is relative. The more you spend, the less weight they usually have. Some very expensive 29ers might be lighter than a cross bike or hybrid just because they are made of a lighter material and speced with more expensive lighter weight parts. My steel Redline cross bike is heavier than my Klein hard tail mountain bike for instance. Another thing you may want to consider is that if you are riding off road, you may want good clearance from your top tube, so that may be an issue for you with a regular triangle bike; I just don't know what you are looking at.

Also, I don't know how tall you are, but 29ers used off road may not be the best best for a short woman or man.

spoke

Thanks spoke, that makes sense. I'm just pondering what I'd want as an ideal second bike to my roadie. My crazy bouldering days are behind me (tho when I did that, it was on a Jamis unsuspended what would now be called hybrid which nearly broke arms a few times I'm sure). I don't like being confined to pavement though and I'd probably use the second on my commute which is all of 6 miles round trip, but does need to get jammed on and off the train. I've taken my road bike before, but I feel like it's more delicate and could get damaged if I can't carry it precisely up the narrow stairs. As for the height issues, I have much more reach issues- I'd probably do best with WSD but have always had mens/unisex bikes and just had my seat really high. Good food for thought tho- I've been perusing craigslist and the sales since people seem to be offloading right now.