Which 20 to 35mm prime lense

Hi!

Sold the nice Nikkor 12-24mm f4 for money matters, bulk and wheight.

Now, I miss something wider than the 50mm 1.8 e-serie.

I am actual using a D40 paired with D70. I will soon replace the latter for the D300 for birding and outdoor photography while keeping the D40 for low-light and hiking.

One of these days, I might change the D40 if an affordable and portable FF option is released.

That is to say, I woul like a «wide» prime between 20 and 35mm that could :

• be use on DX and FX (even if it is not really wide on DX)

• light

• fast

• Af on motor less bodies

• less than 1000$

According to my actual research, the Sigma 24mm macro and the Nikon 35mm f2 would be the best options / compromises.

Any advices and experiences to share?