I´ve been to Valencia last year and to Madrid last Week.
In December Madrid can be cold, Valencia will be warm.
Both cities have a lot of spots. But in Valencia there is a lot of bust.
If you´re in Madrid go to WELCOME or SKATELAND Skateshop. Both are in the same area. Talk to the guys...they are super nice and they´ll help you.
To meet up Skaters in Madrid there are three Ledgespots..."congreso de los diputados", "recoletos" and "principe pio".
In Valencia it´s easy to find skaters and spots. Valencia is small and the city centre has a lot of spots. If you just cruise around you´ll find some spots. If you want to meet up skaters go to the little skatepark (some ledges, banks, flatrail...) in the dried out river called "turia".