My ideal combinaton is 35mm + short tele + 20 (or 21mm).

That said, if you're not adding anything else wider than your 50mm, I'd go for the quite nice 24mm 2.8 (or even 28mm).
The jump from 50mm to 20mm or less would be too much.

I also have and like the Vivitar/Tokina 17mm (and find it far better than the Tamron 17mm many reccomend), but for the reasons above, I certainly wouldn't get it as my only wide...