Oak is the most durable choice for your floor. If you have young children or dogs with nails, oak is a great option. The stuff is pretty bulletproof. You need to be careful with Oak because it can go yellow very easily! Below is a picture of oak flooring...
Now the way the log is sawn makes a difference as to how the graining will look.
Quartersawn boards have two advantages: they are more resistant against warping with changes in moisture and, while shrinkage can occur, it is less troublesome.
The second advantage of quartersawn wood is the decorative pattern on the board, although this depends on the timber species. Since in quartersawn wood the saw cuts across the growth rings, the visible grain is much straighter; it is this evenness of the grain that gives quartersawn wood its greater stability.
Below is a picture of quartersawn oak.
Rift sawn oak is the rarest cut due to the amount of waste, but at my job, it is the most requested. This will definitely give you a modern feel. In my experience, you should keep your planks to 4" in width because the wider it gets, the more likely you will have some rays sneak up on you. Below is a pic of rift sawn oak.
Done with oak, now let's talk about walnut. Without a doubt, this is my favorite, timeless wood floor. I love it when it just has a natural stain. The cathedralling effect of the grain is something you won't be able to replicate. However, walnut is softer than oak, so this flooring isn't for everyone. Below is a pic of a walnut floor.
As with oak, there are different types of walnut flooring. If you are more traditional, or just want to save a little money, character grade walnut is the way to go. This means that there will be knots in the planks. If you are more modern, and have a little bit more to spend, you would want select grade walnut. These are the planks without any knots. It truly is beautiful either way.
Photo credits: http://www.royaloakfloors.com.au/gallery.html, http://barnyardgazette.blogspot.com/2007/09/what-is-quarter-sawn-oak.html, http://www.penningtonhardwoods.com/slidesho/ss_qswo.htm, http://www.keimlumber.com,