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Greetings and Salutations. Like others have said, I don't think much has been said within canon. I can't say I've ever really given the architecture much thought in my games either. But, for my opinion (most of which I'm making up off the top of my head, based and expanded upon from my other pre-existing notions of Elves and Dwarves) ...

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Elves
First, I've never been a fan of the elves are all tree-loving hippies. So I won't be going with trees and nature, etc.

Some of it I can see being kind of Japanese-esque. I view them as liking curved edges, so possibly a roof like this: http://pictures.4ever.eu/buildings/japa ... use-164990 I'm also fond of the concept of them using things such as paper doors.

Designs will be carved into the actual buildings. An interesting shape, an ornate carving into the frame, etc. Items such as gold, silver, and jewelry don't have much meaning. These can be bought. The beauty comes from the skill taken to craft it and the history that lies within. So older buildings will often have the most meaning because they have the most history.

While there are wide main roads, many passageways will be narrow with buildings close together forming a close/intimate type of feel. The narrow passages are only big enough for a single person to walk. This includes narrow bridges over rivers and other hazards. Elves believe in grace/balance and patience, which these help reflect. They also serves as a basic defensive measure.

Dwarves
Subterranean, as many have already noted.
Though thought of as a city built underground, their buildings are actually built within and from the earth itself (so effectively one piece with the ground, not something built on it). In this way, a Dwarven city can be viewed as just one giant and flowing structure (instead of several buildings in one city, each building is literally a part of the city). As such, each Dwarf is part of a larger whole, no matter how far apart.

Their caverns are far larger than needed. Very roomy and very showy. Giants can easily enter and walk their streets. If the Dwarves ever run out of room they can just carve out more space. Because these cities are carved from the earth, cities often have more levels than they are long/wide like surface cities. They can simply expand down, instead of just out. Each level may be different sized, as long as it fits the needs for that level and retains general structural integrity.

Buildings are utilitarian for the most part. Thick and sturdy, designed to last and function instead of look pretty. Standard buildings are tall enough for a human and maybe an Elf to stand with little headroom, but not large enough for a giant (though there are exceptions). Large entry ways (such as into the city, to another level, or even to key buildings) are typically lined with sculpted figures along the entry ways themselves, once again, carved from the very earth as if one piece. They do enjoy decorating things with gems and rare metals, especially their most important buildings and sculptures. Key buildings tend to be built larger (able to fit even giants with ease) and designed to carry acoustics naturally.

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Anyways, just some ideas quickly off the top of my head. Hope they help. Farewell and safe journeys for now.