For £3.99/month you get:
- 150 minutes (giffgaff 150)
- Unlimited texts (giffgaff 500)
- 500 MB data (giffgaff 100MB)
- No on-network calls/texts (giffgaff unlimited for 3 months from most recent purchase)
iD's out-of-bundle charges are excessive: https://www.idmobile.co.uk/help-and-advice/call-charges
Plus you can expect a price change in line with RPI every April.
iD mobile runs on the Three network, so is reliant on 3G. It is no good in poor 3G areas or for anyone with basic phones requiring a 2G signal.
It's also a 1 month contract deal, so there is a credit check involved
So really, there's no comparison.