As someone who up until recently was relying quite heavily on Jumper cables due to a bad battery and lack of funds to replace it, I have to emphasise the following:
The jumper cables were cheapo specials I bought in a supermarket, and were only just long enough for me to get the battery connected to the "jump" car if they were literally bumper to bumper. Longer ones are better, simply because you don't need to do this. Pushing a dead car off the drive in freezing rain is no fun, particularly if you then have to spend a further 5-10 mins in the rain fiddling with the connections to make them work!
As already noted by others, the higher the gage, the less time you'll spend waiting for your battery to be ready. Vital in winter. Might save whoever you're having to jump off a little petrol and time too.
If, like me, you're constantly having to jump the car, or conversely you're doing it for the first time having had little sleep and in the dark, you may accidentally do what I did one cold morning and cross the cables.
This is not advised for several reasons, least of which is the speed at which the metal clamps heat up. Thankfully I was protected from a major shock by the rapidly melting insulation on the actual clamps.
Additionally, as it was just a borrowed oversized battery I was jumping off I didn't risk anyone or anything else other than myself and my car with my idiocy.
Unfortunately, as the insulation was melting, I still got a shock and additionally a very nasty burn to my hand for my trouble. I'd definitely recommend completely skipping anything with shrink-wrap style insulation and go for something with a fully plastic handle surrounding the clamps, or something with very thick insulation on the handles. You'll thank me if you ever do something stupid with them, like I did.