Nifty provides slightly better translations than Google, but I'd call them complimentary. Personally I'd recommend Google as primary translation aid because it's decent enough and it's far more user friendly.
http://www.google.com/translate_tools?hl=en
Go to the page above and look for the Japanese to English link. Drag that link to your browser's toolbar. Congrats, you just got a single-click translation tool on your toolbar. Just click it when you're on a Japanese site and Google will automatically translate the page. You can't do this with Nifty.
Another useful Google feature is that if you keep a mouse pointer hovering over a translated sentence it will display the original Japanese above the text. This can be helpful for a few reasons.
If your language settings are off on your OS you might sometimes see garbled characters when you visit a Japanese site. Both Google and Nifty will understand and translate the page for you. But with the Google mouse trick you can now see the ungarbled original Japanese.
The Firefox translation add-on Rikaichan (which translates words, not full texts) goes hand-in-glove with this Google feature. You don't need to switch back to the original untranslated page, like you would with Nifty if you want to use Rikaichan (and you're out of luck if it's a garbled page). So Google provides the context right in front of you and with Rikaichan you get a choice of words from which you can pick what best suits the context. The addon also helps in identifying specific Japanese words. Just copy the words you're interested in and search with Google. Note that Rikaichan becomes even more useful if you have learned Katakana and Hiragana.
This Google feature is actually there so people can send in suggestions for better translations. So if you think your Japanese is good enough, help Google improve.
