S5 is not in the same category as 535xi.
If you don't need 4 doors, get a coupe. The M3 coupe, or upcoming M4, are what you should be looking at. S5 is beautiful, but it doesn't compare in performance/handling to the M3 and likely the gap will widen even more when M4 comes out.
I'd suggest waiting a bit and getting into the new M4.
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