The answer depends on your application, the maximum trace length & the layer materials.  

Say your board material is FR-4 for all layers. For a 50 Ohm single ended microstrip on the 14 mil layer you can expect a loss of about 0.1 dB/inch at 2 GHz, mostly arising from the dielectric. The 32 mil substrate will exhibit a slightly lower loss - a second order effect, since an ideal TEM line has a dielectric loss that is independent of geometry, depending only on frequency and tan(δ). However the 50 Ohm line width will be ~ double that in the 14 mil case, which can impact routing. Edge-coupled differential lines are more susceptible to odd mode dielectric loss as the electric field lines get scrunched up between the traces, and the current crowds the sides of the conductors.

In short, at 2 GHz you don't have any serious problems. The same FR-4 for a 36 inch 10 Gbps serial backplane would become a formidable challenge when you throw in connectors.

M.G.Rajan
www.eecalc.com