The best is probably WAYW, in that it will let you see all sorts of different combinations, so you can decide what you like. In short, the major considerations are design, color, and type of leather. A balmoral shoe is dressy and typically worn with suits. The ultimate suit shoe is the plain captoe balmoral:
Notice that it is sleek and elegant and somewhat austere. In black, it would be the most formal type of shoe you would wear with a suit. (Tuxedos are another matter.) Then you have bluchers, which are more casual. These are also called derbies:
The difference in balmorals and bluchers lies in how they're put together. Compare: Balmoral:
Balmorals have what is known as "closed lacing," meaning the quarters -- the sections with the laces -- are stitched beneath the vamp, the forward part of the shoe. This gives a sleeker profile than offered by bluchers, which are built in the opposite fashion and said to have "open lacing." In general, sleeker = dressier. By the same logic, a shoe with a thick double sole will be less dressy than a shoe with a thin single sole. Some guys wear bluchers with suits, some don't. The success of the pairing depends on both the cut of the suit and the style of the shoe. Sometimes a hefty clodhopper can make for an interesting juxtaposition with a sleek suit. But sometimes it just looks clunky. You'll have to eyeball it. Aside from the design of the shoe, the color, amount of decoration, and material also come into play. Black shoes are more formal than brown, and dark brown is a bit dressier than light brown. But this can be a subtle distinction. A black Kenneth Cole square-toed blucher is in no way dressier than a sleek dark brown balmoral. In general, though, the black version of a shoe is dressier than its brown counterpart. Brown traditionally was not worn at night or to serious events such as weddings, interviews, funerals, etc. Similarly, suede shoes are less dressy than calf. You wouldn't wear suede to most job interviews, though suede can work well with everything from suits to shirt and khakis. The final factor to consider is how much decoration a shoe has. The more broguing -- decorative holes -- the less dressy the shoe is. A black wingtip balmoral is less dressy than a black captoe balmoral, though many people think the opposite these days. If you've made it through all this and still want to learn more, check out A Harris' post here.
It covers the above plus much more, such that I thought a simplified explanation might be a little less overwhelming to newcomers. By the way, you can click the top two pics to see more examples of balmorals and derbies. Doing so will take you to the site of SkyValet, a great little shop in the D.C. area.