It really depends on what you prefer. Maple is considered a softer wood compared with Walnut. Also Maple usually have less grain shown through when stained.

Hard Maple floors are about a 1450 Janka hardness, they dont stain very well and have very little grain showing. Black Walnut rates a 1010 on Janka hardness, it stains very well and is also world renowned for its grain patterns (think burled walnut etc). I would certainly go with the Black Walnut. Here are some shots of an 8" Walnut floor we just sent to another Houzz user. Let me know if you have any walnut questions or needs. joel@oakandbroad.com

Its all about personal choices - likes or dislikes However that being said please note that walnut is a more durable wood.

J Design Group, Modern Interior Design Firm in Miami, Florida - Contemporary Miami Interior Designers http://www.JDesignGroup.com

I chose walnut and I LOVE my handscraped walnut floors. But, what color will work in your home. Even natural walnut will be a medium tone so if you want light, go with maple.

I agree with SoCal...
The difference between the two is literally night & day, in color, grain, etc.
You can achieve a more contemporary "look" with Walnut, but to do it, you need to lean toward a higher grade or a rift sawn cut to limit the regular grain patterns, which is where most of us will agree, is where the natural beauty of Walnut is.
Aside from that, as SoCal suggests,,, color & the rest of the interior needs to be the primary consideration. One last caveat... one of the things that makes Walnut a viable choice as flooring, is to lean in the direction of at least SOME degree of distressed or hand-scraped style finish. It doesn't have to be "barnyard chic"... but even a very LIGHT or SUBTLE approach is helpful... along with a high quality topcoat/finish.