Priority tags is a benefit I value a lot. While it's true that there are so many bags tagged priority nowadays, being able to guarantee that your bags will be among the first half to come out still makes a difference. Your luggage runs the risk of being one of the last few to shoot out of the carousel if it's tagged without priority...

In short, lounge access and priority tags are the biggest perks of SL over GR. The rest are rather trivial IMO.