OP,

It all depends on your preferences. Some points to note.

1. AFAIK, the geometry on the 2013, 2014 (certainly) and 2015 (probably) carbon Sirrus models is identical. They are all, without exception, Roubaix frames with flat bars slapped on them. So, one from one year isn't going to be 'racier' than another, etc. If you're happy with that adaptation, then they are all going to be similar re. handling etc.

2. You can't retrofit discs to a 2013 Sirrus Pro; ditto calipers to the later ones.

3. You can (probably) change the drivetrain to electronic on a 2015.

4. The 2013 Pro, as you note, has 10r carbon; it will be a little lighter/stiffer than 8r.

That's about it (price etc. aside); choose your priorities/buy the one that floats your boat. They're all lovely bikes, if they suit/fit, I'm sure.