Is spectrum of sunlight the same in the summer as in the winter?
As you suggest, longer the path, larger the proportion of scattered radiation. Since the shorter wavelengths are more strongly affected by the Rayleigh scattering, sunlight appears more red when the sun is low.

Lower solar elevation angle will also result in longer path through the ozone layer, and hence stronger absorption at the UV-B wavelength range. Together with the Rayleigh scattering, this will result in somewhat less UV-B compared to UV-A, all other things being equal. 

The online version of NCAR's TUV radiative transfer model can evaluate the clear sky radiation for a given location and time. At noon, 45° north, I get UV-B/UV-A ratio of about 0.03 for 30th June and 0.01 for 30th January.

In addition to the sun elevation, the radiative fluxes will be affected by a number of factors like clouds, surface albedo (snow!), aerosol extinction, and variations in the ozone layer.