I'm building up a winterbike. I got a an M4 Stumpjumper from here and am about to build it up. I got a pair of flat Race Face evolve bars to go on it.

Will I find a huge difference in the handling of the bike. I've not ridden a hard tail in years and every full susser I've had has had riser bars.

Should I stick to what I've ridden with before? or go flat? Whats the worst that could happen?

To me the only difference between risers and flats is the bar height. Flats and risers can be the same width, sweep etc just at a different height. If you don't like the reduced height with a flat bar, stick some spacers under the stem or go back to risers.

If the sweep's the same as your riser bar you won't notice any difference.

I wondered if the rise had a 'spring' in it?

no 'spring', just a lot of marketing bull.

I wondered if the rise had and 'spring' in it?

I would imagine the material the bar is made of and whether its normal or OS will have a bigger difference