There are some things that you have to consider with routers.
differences between 802.11a/b/g routers:
802.11a: Just like the g version, however the only difference is that it has a frequency of 5ghz... but you might not be using this because generally only b or g are used.
802.11b: Like Mortalkrab said it has a maximum data transfer rate of 11mb, which might be more than enough to more people because chances are that your current internet speed is about 7mb or so (if you have broadband)
But another thing that you have to remember is that this has a shorter range, so if you are living in small space (or are going to use it in a small space) chances of people stealing your internet (getting WEP cracked). But if you want you can also extend the range through products like wireless g-range expander and so on.
Also b and g have a frequency of 2.4Ghz (just like your cordless phone... not the analog 900mhz ones though) so it might interfear a little bit with your phone or vice versa.
802.11g: These have a maximum data transfer rate of about 54mb, but have a wider range (due to newer technology). So chances of people stealing your internet is higher. However, g is also backwards compatible with b, so if you have a device (like a laptop) that has a b wireless networking card, then you'll still be able to use g. But the prices for this kind of router might also be high.
In the end it just comes up to what you need, so its not necessary that when you actually use the routers one kind of router is faster than the other.