The Diverge is light years ahead and has drop bars, which I prefer. So you'll be able to ride on the hoods for most of the time, but get aero in the drops if you need the extra aero while descending or whatever.

The Diverge has a 50/34 front and 11-32 rear -- which the 50 is going to where you are primarily, and should allow you to keep up with everyone on the road bikes.
The Sirrus, on the other hand has a 48/32 -- which isn't going to be THAT much different, but it is still going to have a lower top speed.
Couple that with the fact that it's going to be easier to get lower on the diverge as your fitness improves and your ability allows, if you did want to take a break from fighting the wind on a windy day/ride.

The Diverge is just a better all-around, better built bike that is capable of much more than the Sirrus in my opinion.

As far as fit, make sure the bike shop fits you, and make sure you're comfortable. If they're not able to do this, then find another bike shop. The should be able to adjust the handlebars for you to an angle that you're comfortable with (where you can adjust as fitness improves or simply leave how it is), or flip the stem over for you to get you more comfortable, etc. Many many things they can adjust to dial that in for you, and IMO any reputable bike shop will do that all for free when you are serious about making a purchase.

I think if you get the Sirrus, you'll always be wishing you got the Diverge. But I don't believe the opposite to be true. The Diverge is truly a machine of beauty and very capable of tackling any gravel path or group ride you can throw at it.