GiffGaff runs on O2 network, which I'm on too (with Tesco) so I know their coverage is okay locally, Three is ok coverage, too.
The £10-top-up package on Giffgaff looks better, unlimited texts & free phone calls within network for a while after top up, too. Around here more people seem to have Three for their teens & rate it highly. Three means a mere 300 free texts/month with £10 top up; can't teens easily send more than 10 texts a day? That's why I'm thinking to go with Giffgaff (DS is 12, btw).
Thoughts?