29er basically a mountain bike with slightly larger 29 inch wheels than the usual 26, They're pretty much the same 700c diameter as your Defy but able to take the wider tyres such as you have on your 700 for a less jarring ride. I had an older than yours Trek 700 model and liked it a lot as a messing about bike, a quite nippy all rounder for its weight and rugged nature, but sadly some thieving toerag took a liking to it too when my son was using it for work.
It does sound as if you don't really need a mountain bike for the type of riding you describe here; but there's nothing wrong with them; and you could sell both of your bikes and buy a new much lighter mountain bike and make it road friendly as you've done with the Trek and not be out of the ordinary at all - the problem here is that the majority tend to offer suspension and by the sound of it you don't need that, front or rear. Assuming you keep the trek & want something else more designed for road and light trail, then either a hybrid (the cannondale quick range:
http://www.edinburghbicycle.com/browse?keywordFilter=hybrid bike&bGOmainKeyword=Search) or cyclo cross (CX) (e.g.
http://www.edinburghbicycle.com/products/revolution-cross-13) would eat up the road miles better and be perfectly happy on your weekend rides, they are also designed to take mudguards & a rear rack in a way the Giant isn't really.
The CX obviously gives more hand positions and the opportunity of a lower more aerodynamic riding position in bad weather straight out of the box but bar ends on a hybrid would give you similar options to change grip and stretch out a little bit more - back permitting. Both types are going to have gearing more akin to your Trek (3x7+) than the Giant (2x8ish) which will make the hills easier and put your heart rate up less. Again from my own experience I'd suggest the CX over the hybrid as I find I can assume a more natural and stable; weight forward; uphill position on a bike with drops and feel more in control riding on the hoods than I do pulling uphill on a flat/hybrid bar.
Your best bet is to take a bit of time in a few decent bike shops throwing your leg over different styles of bike to see which one grabs you best. You could take the Giant in and chat about adjusting your position on it from a comfort point of view (although racing road bike geometry does stretch you out more than any other) and whether it might be viable to put a triple chainring on the front to give you a wider range of gears without breaking the bank. Tho (stuck record
) if you get £400 for the Defy you could buy a Revolution Cross 13 new with change to spare to accessorise it in their current sale.
