How aero is Focus Izalco Max against other aero bikes like Madone, Venge and Propel?
Any differences between all the 'aero' bikes will be very small. At worst it could be fractionally less aero in some circumstances but no-one could notice. IMVHO I wouldn't worry about it.
Far more important is that fits properly and you enjoy riding it.
I am thinking of buying Propel or Izalco max. I am just not sure how much faster is Propel frame than Izalco? So is it worth going for Izalco becouse it is more all-around bike.
I'd go for which ever one fits you best. As Simon says the differences are very small once you're looking at the top 10 bikes. Being more comfortable and better positioned is going to give you better performance than saving 5 watts on the bike being more aero.