I would go with the Allez... it's a very nice bike...
if you found the 56 to be nice, the smaller size will be more fun as you progress...
as for 'stack', that's just a number...
The allez seems to have a 7 deg stem (that's the angle off 90 degree to the steerer of the fork), which comes 'stock' set in the 'down' position.
If you have the dealer 'flip' the stem, you will immediately have a 1.5 cm higher 'realized' stack to the handlbars... reference the
Habanero stem angle chart
You can always 'flip' it back or get another stem, different angle, and so on...
It's your 1st bike, if you really get hooked on riding, then after some time you'll develop your own 'gotta have' s for any future bike.
The Allez is a nice bike - more than good enough to really take you deep into the rabbit hole of sport cycling... LOL!
save up your dinars... you're gonna need them if the 'itch' need scratchin.
LOL!
Ride On
Yuri