I know this post has been around a while, so I will just add some useful info for anyone that is thinking about getting some jumper cables.
first, what I recommend is at lease 4 gauge, 20 foot, or better jumper cables. If you have the money, try to get all copper wire jumper cables. If you don't have the money for all copper, then get at least 2 gauge cables. The reason I say this, most jumper cables sold now, especially the cheaper ones, ane what they call copper clad. What that means is, the jumper cables are made out of an aluminum cable/wire, with a very thin, copper coating on the outside. More then 99% of the wire is aluminum. Because of this, a 4 gauge all copper wire will give about the same amps as a 2 gauge copper clad aluminum wire.
The other reason why the all copper wires are better is because the all copper ones are more durable then the copper clad versions. While I am in no way saying that the copper clad version is not durable at all, the aluminum in it can corrode over time, where this is less likely to happen with the all copper ones.
The reasons that I say to get at least 4 gauge is this. Once a vehicle is started, the alternator should produce enough power to keep the vehicle running, no matter what kind of condition the battery is in. If you get a vehicle that the battery is just about completely dead, or does not hold much of a charge, the 4 gauge ones should be able to get that vehicle started with no problem, and then the alternator should continue to charge the battery, along with keep the vehicle running. But if you use some cheap 10 or 12 gauge cables, you will have to wait a while for the battery to charge up enough to start the vehicle, before it will start. Those thin jumper cables will add very little power to what is in the vehicle, so you have to have the power charged up in the battery to get it started. If that battery does not hold a charge, then the thin, cheap, 10 or 12 gauge ones, will never be able to get the other vehicle started because they can not transfer enough amps to get it started. So you will want something that will be able to get the vehicle started, no matter the condition of the battery.
And as other have stated, the 20 foot length should give you plenty of length to get to the battery on both vehicles. You never want to be stuck in a position where you have to push a vehicle, and move it, just so you can get another vehicle close enough to jump it. This can especially be problematic if you are on a hill, or in the snow, etc.
As for any of those jumper boxes that plug into your cigarette lighter. Those will only be good if you have a slightly low battery, and have plenty of time to wait for the power to be transferred from the box to the battery. Also, dont forget, if you have to turn your ignition key to the on position to get that socket to work, that also means that you are going to waste a lot of that power, to power the stuff in the car when the switch is turned on. Those type of jumpers, while good for a slightly low battery, and if have the time (which can be a good 20 minutes or more). But, if your battery is, say half dead, or more, then those types will never give your battery enough power to get it started.
Think of it this way. Most regular jump starters, whether it be one that you connect straight to the battery, or though the cigarette lighter socket in the car, all have what they call SLA batteries in them. What this is, is basically, the same exact type of battery that is in your car, but in a spill proof container. SLA stands for, sealed lead acid. Now take into consideration that the battery is also in a carrying case with lots of plastic around it too, plus any circuitry, etc. Now compare that to the size of your battery in the vehicle. It (the jumper) is many times smaller, which just goes to show you, just how little they have in power. If you need numbers, some of those jumpers can hold as little as 7ah of power.. while a small car battery will start at around 50ah of power. The higher the number, the more power it can store. That means, a vehicles car battery can be 7 times more powerful, and can around as much as 14 times more powerful for larger vehicle. This is why they can be useful, but also is very limited too.
This is also the reason why, you can not rely on them, if your car battery is too dead too. This is a good reason why, you will not be able to replace a good set of jumper cables with a cheap jumper box. and with jumper cables, as long as you have a vehicle there that is running, you have an, almost, unlimited source of power. But jumper boxes.. Once the battery is dead, it is useless until you spend a few hours recharging it. Not to mention, because it is a battery in it, the jumper boxes, are only good as long as the battery is good too. You accidentally hit the button and turn the light on.. Don't notice it.. guess what.. When you go to use it, that jumper box is going to be dead.
In the end, while jumper boxes do have their uses, as do cheap/thin jumper cables, you really can not replace a good set of jumper cables with either of them. At the least, you should have a good set.. and if you still want the jumper box, get that in addition to the jumper cables.