I know both spots.
In December expect better winds in Guajiru.
For freestyle you want flat water, which the small lagoon in Uruau offers, but can be very small if there is no rain...lagoons dry or get smaller out in dry season. It is already small, but then it will be tiny...and then dangerous if you crash, plus 2 riders and it will be pact.
Guajiru has no flat water, it is ocean riding, which is wavy and choppy. There is two lagoons upwind, you need a buggy or 4x4 to get there (which can be rented or used as taxi).
But, also depends on tides and rain if they are big enough.
The downwinder from Guajiru to Mundau is very nice, specially when a swell is coming in.
Mundau also has a big lagoon (river mouth), but it is gustier there...but on the ocean on low tide you will find very nice flat water pools...be careful with the reefs and rocks, which you can easily see.
All spot info on
www.splocs.com
Enjoy!