No point in a mountain bike at all if you aren't planning on getting even a little muddy. My single speed on road racer wheels does a little off road (tow paths and cinder track) now and then and that's fine unless super wet.
29er wheels are a good recommendation for the type of riding you have described BUT that's just adapting a mountain bike to be more like a hybrid really. Something like this hybrid Norco may be more apt for you...
http://www.evanscycles.com/products/norco/vfr-3-2013-hybrid-bike-ec041287Or, you could always go even further and step into cyclecross/touring territory. Nice, fast and more importantly...easy. The riding position may be a little odd at first but you get used to it very quickly and get more confident than you perhaps would be whilst sat bolt upright as you are with a hybrid and mountain bike.
This Roux may be a little out of your price range but looks to be a well spec. entry level cyclocross
http://www.evanscycles.com/products/roux/conquest-2300-2013-cyclocross-bike-ec051745Have a read at the description of the 'cyclecross' genre at the top of that page, I feel it fits exactly what you are looking for.
*EDIT* Re-read what you wrote. Who advised you NOT to get a hybrid? That was silly advice. If you go with the GT for tow paths and tiny bits of off-road, all it is going to do is cover you in sh!t unless you have really good, well fitting mudguards but even then you will get splashed. A cyclocross or hybrid with a narrower tyre will all but eliminate this with small splash guards on in winter
Read: You will ride it more as it will be much less miserable Typically a cyclo should be 700c wheel size (like a road bike) and a hybrid may be 700, 29 or 26.
