Get Leica 28mm at B&H Photo.
The revised lenses are mainly about bringing the physical design to current standards so that they are similar to the 28/1.4 Summilux and various other Leica M wides. Both 28mm lenses also incorporate optical tweaks, for claimed higher performance.
I expect to have both lenses to test sometime in February, on the M240 for sure, and if things go well the Leica SL may be at hand.
The 28mm focal length is a favorite of mine, and the 28/2.8 Elmarit is a diminutive lens, really nice for 'carry'.
It looks like the 28/2 Summicron is the clear winner at f/2.8, and of course it also has an f/2 for blurring the background. So if those are key apertures, the 28/2 looks to be the choice.
Update: is Leica’s data sheet wrong? Perhaps the f/2.8 and f/5.6 graphs are reversed (as of 28 Jan 2016)? NOW CONFIRMED by Leica, the charts as posted by Leica had the f/2.8 and f/5.6 graphs swapped. Presumably Leica will correct the datasheet soon.