If you want to remove the lockring around older bottom brackets then you want HCW-5. If you want to remove the lockring from a fixed gear wheel you do NOT want HCW-5(which can make the process an unnecessarily miserable experience). The HCW-17 will work fine for that though. I have the Hozan version which works really well. Even the Pedro's Trixie tool has a lockring spanner if you want something portable. However, ideally what you want if you are going to change cogs frequently is this:

http://www.amazon.com/Shimano-TL-SR2.../dp/B002P78X0A

I think there are places where you can buy it cheaper than Amazon though. Rotafixing is good in a pinch but a proper chain whip works much better and is much nicer to the threads of your hub. The shimano chain whip has a built in lockring wrench.