Mondrian pisses me off - they are one of the most expensive resorts but they charged for valet since day one. Ludicrous for the rates they ask. Four Seasons is VERY nice, their valet is excellent and friendly plus they have the excellent pool at the Mandalay you also get access to in addition to the private Four Seasons pool which is just kind of simple.

I like the suites at mandarin and they have a very nice lounge to listen to music on the floor where registration located.
I have been to Mandarin quite a few times never had to pay for valet except for tipping as you would do at any other hotel.
Perhaps that is something that was just changed.
Four Seasons is nice but you can see the difference in the rooms just check out oyster.com it will show you pics of the rooms.
Enjoy and have fun!

By your female companion coming to visit you. She will not have to worry about a key card to access the elevator or get past security.  

Mandarin is very beautiful but you will have to meet her in the lobby or lounge on the 23rd floor as you need key card access to make the elevator function to the guest floors.  

I enjoy  visiting both properties for different reasons.  

a second home, my choice would be Mandarin. I have stayed in a few different suites and they are all very roomy, comfortable and some have a fantastic view of the strip. Which is great when you are in coitus with your date against it. Hahaha. The bathrooms are large and roomy.

I never had an issue with meeting my dates in the lobby or at the bar on the 23rd floor. I like the secluded effect of the hotel, since they are just set back of the strip. Unless things have changed recently, I have only tipped for valet and always have given my date valet tip money.

Have fun wherever you stay.