I am a fan of buying cordless tools from the larger brands that make a lot of different tools that work with the same system.
Many times the unique accessory tool that is available for your cordless kit ends up being the most used tool.
A recent addition of the C3 Craftsman Inflator has highlighted the choice of system to me. Many Battery systems do not even offer an inflator. My inflator is getting even more use than my drill. I have several cars, and several bicycles and I have always liked to keep the tires topped off. The C3 inflator is just so handy and easy to use that you will never procrastinate checking tire pressure again (it is actually kind of fun).
Even if I am only planning on using a Drill driver and the saber saw (really handy for trimming trees around the yard or getting branches while camping). I want to have the option of buying other tools.
Here is kind of an interesting thought also. I want to buy tools that are from a major brand with a lot of distribution on sites like eBay and Amazon. This way as my tool gets older and the manufacturer moves to a different standard (usually a higher voltage standard) I will still be able to get more batteries for my tools.
Two of the Systems that are my favorite as of Thanksgiving 2013 are the Ryobi 18v One+ system and the Craftsman 19.2V system. I have the 19.2v Craftsman system and I just recommended the Ryobi One+ system to my son because of his high volume portable inflator usage. While both of these are the older Stem style of battery they both have a very large group of tools built to work with them. They have invested a lot in the design and manufacture of the different tools to work with theses systems.
Click Here! to check out the Ex Factor.