The
L is reportedly canons sharpest zoom lens out there. I've it's big brother and that is sooo sharp and astonishing I still dont believe how well it performs. I dont see how it can be sharper, but if it is then... wowee

Also there will be almost zero distortion and fringing.
The 70-300 IS is a good lens. It is not as sharp as the
L nor is the colour rendition as rich and true (not that its bad, but you can see the difference). If you compare the
edges of the frame against a similar shot from the
L then you'll see how the quality falls off.
IS can be very good, but it is not a golden bullet solution. If you are a tripod user or shoot outside in good light then IS doesnt really help much. It will however generally provide a higher sharp/blurry hit rate.
What you need to do is take your cam down to your local Jessops /
London Camera Exchange / X-----X and try out the lenses. Take lots of shots of near far, large f (10+) small f and then go home and compare and contrast.
In short there isnt an easy answer, except SAVE and get the
L IS version.