If I intended to go full frame in future, ill definitely own the 35, 50, 85 etc of all 1.8 ones out there by Nikon instead of combinations of telephoto and prime. These would keep the cost low and good collection of lenses to use with both DX and FX formats. Future safe.
Alternate option are the third-party telephoto lenses which have decent 2.8 for bokeh and low-light assistance. Won't comment on the quality comaprison, since i have never tried one but i'm sure they are bang for the buck.
So in the end its upto what you want and what suits your need.