Just get the strand in walnut. It is arguably AE's best shoe. It looks fantastic with jeans and suits (or really anything navy to mid-grey). However, I would not wear it with khakis as tan and khaki just don't look that great.
1. Pick whatever feels the most comfortable. You will be on your feet a good portion of the day.
2. A derby style shoe is more casual (as previously mentioned), but the idea that walnut strands don't look good with a nice pair of dark jeans is silly. It depends on the environment you are in. To some, any Allen Edmonds shoe with brogueing will seem very fancy to the general public (even though traditionally it is seen as more casual). To others, no one will give the...
This is the one you want. They also have a longer version.
Wait until one of the semi-annual sales and you can get it for like 30% off. I got one in charcoal and love it.