RF Design - Microstrip vs. Lumped Components
I'm not an expert in RF, but my guess is that the following tradeoffs come into play:

lumped components:


you don't have to design them
as long as they're physically much smaller than the wavelength, they should work well
except for high-power or high-voltage, should be available in 0402 or even 0201
if you need to change frequencies, you can do so by changing the BOM + can leave the circuit layout unchanged


microstrip elements:


tolerance is controlled by geometry, so can be much better than lumped components (esp. relative tolerances) with higher-Q/sharper rolloff
you can do all sorts of things you can't do with lumped components (e.g. resonators, quarter-wavelength transformers, directional couplers, etc)


Wavelength is definitely the key -- 1GHz has a wavelength of about 30cm in air (less in a high-K material like FR4). I've heard various rules of thumb like lambda/2pi, lambda/10, lambda/8, but if your board traces have dimensions of >3cm you probably need to deal w/ transmission line effects for signals > 1GHz.