Nikon Prime Lenses

[SwissJon]

I'm looking to expand my Prime lens collection beyond the solitary one I have. I would like to start at wider angle lenses because I shoot at that end of the scale mostly..

There appear to be 2 on the Market:

Nikkor 20mm f/2.8D AF(Which I assume is DX)
Sigma 20mm f/1.8 EX DG (Which appears to be FX)

Now here in Switzerland the Nikkor is twice the price of the Sigma.. In the UK (Where I am going soon on business) the Nikkor is about the same as the Sigma.

So the question is.. Is the Sigma worth buying? Is it an FX lens? And is the Nikkor really DX only? (i.e. The FF Nikons need to use DX compatibility mode)

[OsmosisStudios]

Both are FX lenses. Nikon only has 2 DX primes: the 35/1.8 and 85/3.5.