I'm thinking about which SIM-only deal to get as well. Here are my thoughts...
From 6th November GiffGaff (who use O2) will be:
£10 for 250 mins + unlimited texts + 1GB mobile internet (tethering allowed)
£12 for 250 mins + unlimited texts + unlimited mobile internet (tethering not allowed)
(GiffGaff did a consultation on their price change recently and you can find a slide show on this page.)
Virgin (who use Everything Everywhere (T-Mobile/Orange)) look like the closest competitor:
£10 for 250 mins + unlimited texts + 1GB mobile web
£12 for 1200 mins + unlimited texts + 1GB mobile web
£12 for 125 mins + unlimited texts + unlimited mobile web
It looks like Virgin's "unlimited mobile web" is subject to a 10GB monthly limit after which speed is reduced (during the day) until the end of the month. It's not clear from those T&Cs whether tethering is allowed on Monthly contracts (although a post from a Virgin employee in the support forum suggests that tethering is allowed). It is made clear elsewhere that tethering is not allowed on PAYG contracts.
So, given that I want to be able to tether ocassionally, I reckon I might try the £10 deal on GiffGaff and see how much data I use. If I exceed the 1GB I'll probably switch to the 12GB deal on Virgin (and hope I'm ok with the lower minutes).