Let me throw a couple of thoughts into this discussion about formal versus informal. My experience that the vast majority of people wearing suits just throw anything in their feet as a "dress shoe." There are lots of cheap glued shoes with rubber soles at the bottom of suit slacks. While the readers of this forum certainly know finer footwear, the average person will never notice an Oxford versus a derby and make a decision about the level of formality.
Second, there are some people who use a list of criteria to determine the "formal" shoe and this places shoes that do not meet each criteria onto some scale of "informal." My experience is most of these criteria are based on "Sleekness." Oxfords are sleeker in appearance than derbys. Double oak soles are less sleek than a single oak. Somehow this applies to brogue as the plain cap toe is sleeker than the brogue covered shoe. I was told that I was mistaken that brogue patterns and mutliple layers of a wingtip was less fancy thank a plain cap toe.
This information was particularly frustrating to people like me who have yet to find an Oxford design that fits our feet. I certainly cannot justify a bespoke Oxford just to have a shoe that I can wear with a suit. I, and some others like me, have grown content with a fine leather AE derby shoe with a leather sole with our suits. Sometimes plain; sometimes with brogue. 95% or more of the people do not give a second thought to these "dress shoes." With a good polish, nice shine, proper edge coating, these will be just fine with a suit. Just my opinion based on observation. "Your mileage may vary."
Speaking for myself, I think a split reverse welt and a thick sole do more to make a shoe informal than the open vs close lacing. One thing people don't mention is the level of polish - you can make a shoe more formal by upping the shine, and less formal by keeping it matte.