Samsung 20mm F2.8

Hi,

I have just ordered an NX11 with the standard 18-55mm lens.

I am wondering whether to purchase an additional lens at this point - either the 20mm F2.8 or the 30mm F2.0. (since they are both available for lens than £100 on ebay, I imagine the new 45mm F1.8 will be way more expensive? Any guesses as to the price?)

The benefit of the 30mm seems clear since it is much brighter than the kit lens at that focal length F2.0 vs approx F4.2). I would hope to be able to get some amount of background blurring with this lens. Although it is a shame that the 30mm is not an i-function lens.

However, the benefit of the 20mm F2.8 is less obvious since the kit lens can already do approx F3.6 at a focal length of 20mm.

Is the difference between F3.6 and F2.8 sufficient to make this lens worth having?
Or are there other benefits to be gained from this lens?

Thanks for your input.