please ignore this if you already knew:
as part of the Lufthansa Group, OS is sharing pretty much everything with sister companies Lufthansa and Swiss.
I would dare to say only the logo is different, other than that they have same ticketing system, same prices, same classes etc.

So any of the above websites will give you the exact same options.
Opening an Austrian account gives you access to the exact same platform.

Flight experience is a different aspect.
I find that the flight crew will make more of a difference than to what airline it actually belongs to.
As a general rule, I find OS aircrafts newer and cleaner.

But personally I'm avoiding OS due to their FFP2 policy.
OS is enforcing them, whilst LH and LX are (at this moment) not.

On the ground and in the airport, I wear a FFP2 as often as I can, but I find it hard to bear on a long haul, especially when sleeping.

Miles wise,
If I were you, I'd sacrifice direct flights comfort to miles gain and choose FRA, MUC, ZRH connections as often as I could.
Decent lounges and fairly good airports.
Though, if ever given choice, choose:
1) ZRH
2) FRA
3) MUC

Bavaria (MUC) has special rules which are often the most restrictive of the three.

Also, note that some LX flights are connecting through LGA-YUL which I find a hack, due to short US immigration waiting times in YUL and re-vamped LGA.

I guess I derailed a bit here
good luck!