Will depend completely on how sensitive to the sun your skin is chum. Mine is v sensitive, so I use high spf.
Unfortunately also you can't layer one product over another and expect it to work properly.
I use a face mois. spf 15 too, but carefully only put factor 20-25 on my cheekbones / back of neck on v sunny days. If you use P20 or Ultrasun sunblocks (*brilliant* stuff) note if you even have moisturiser on your hands when you apply them they will not work...it stops them bonding to the skin.
P20 seems a bit harsh for the face, but Ultrasun seems ok. Hope this helps a bit :O)