The two options that are commonly available are the battery back-up sump pump and the water-powered version. Now it is important to mention that these two options should never serve as your primary pump, they are only to be used in the back-up role. The purpose of a back-up sump pump is to be on standby should your primary pump be overwhelmed during a high influx of groundwater or if the power goes out rendering your primary pump useless.
The first question people raise is whether or not they should even invest in a back-up sump pump. It is honestly not for everyone. It is however a great preventative measure to take. Some factors play into whether or not a homeowner should install a back-up. One is primarily an issue of groundwater. How often does your primary pump run? Does it run regularly or only when it rains? Other things to consider are your soil type and whether or not the grade (dirt) slopes away from your house. Because if a lot of water leaches in, you can bet that the chances of your primary pump being overwhelmed or malfunctioning go higher. Another simple question to ask is whether or not your basement is finished or if you have plans to finish it in the future.
So let's say that you've decided to invest in a back-up sump pump. An initial universal cost that you will need to be prepared for is running a separate discharge line which is recommended by most installers. The reason for this is that if the check valve on the other line fails it will just push water back into the pit if the lines are connected. After considering this that leads us to which option to choose: Water-powered or Battery back-up. There are pros and cons to both and so in hope of painting a fair picture of both their characteristics will be laid out.

However, one must consider some of the set backs of a battery back-up. Firstly it is battery powered and therefore it will only last so long. So in the midst of an extended power outage, it will eventually run out of power. Also the batteries themselves have an average life of 5-7 years (but can go bad sooner!) and will then need to be replaced. That is extended maintenance and cost to the homeowner. These are things that one must consider when looking at a battery back-up.
Now onto the water powered back-up. When installing this type of system we normally go with a Guardian system from A.Y. McDonald. These systems are run solely by water which is fed by a 3/4" line that will need to be run from your existing water piping. A great benefit of running on water alone is that it is not reliant on electrical power and therefore will be unaffected by any power outages. It also will run as long as you have water pressure in your lines so you don't have to worry about a battery going dead after hours of pumping. The peace of mind of this invaluable when the lights are out and the rain is pouring down. The reliability of this product is often times worth the extra cost that comes with it.

I hope this gives you a platform to begin a deeper investigation into the worthiness of installing a back-up sump pump as well as the pros and cons of each style. This is not an exhaustive list by any means, but it is a good place to start. Hopefully now you have some of the information you need to make a decision.
Just don't wait until you feel the squish of wet carpet between your toes before considering the practicality of a back-up sump pump!