Pretty straightforward, I think.

The two bikes, currently (e.g. 2017) are pretty similar: all-rounder disc-braked road bikes with decent tire clearance, one with geometry for flat bars (Sirrus) and one for drops (Diverge).

The '18 Diverge has been redesigned -- with some changes/improvements to my mind -- and I expect the Sirrus will follow, possibly even with the 'future shock' on the top one or two versions, either this year or next.

Choose your preferred bar type -- purchase accordingly. Of the two, I'd go with the Sirrus (my main bike is a 2010) because I prefer flat bars/bar ends for road cycling. If I preferred drops -- Diverge.