Cellular Service provider in the Glasgow area

We are looking to setup service with a Cellular Service provider in the Glasgow area.
We currently have AT&T here in FL and need to have a phone setup as soon as we arrive in the area in 6 weeks. Should we do this ahead of time or wait an visit a store when we get there? What is the best company to go with? (we will be bringing 2 iPhones)
Any help or suggestions are appreciated! 

Order pay as you go sims before you arrive and then load them with credit when you get here.
I use Giffgaff which runs on the O2 network, monthly pay as you go "goody bags" are really good value and I have found the coverage to be good. My kids are here on holiday and we ordered a couple of sims for them so that they have unlimited internet, unlimited UK texts and 250 UK minutes for £12.

www.giffgaff.com

will i have to wait to get a UK bank account to load them?

Not sure whether you can use a US c/card online with Giffgaff, why don't you go on to their site and ask the question? Someone in the users forum should be able to tell you.

But you'll need a get a UK bank account and card set up pretty quickly anyway?

Sorry, just remembered that you can buy £ credit at an O2 shop and then use the voucher number online when you register your giffgaff sim. That's what we did before we had a UK debit/credit card.

Hope this info is useful!

thank you for the very helpful info!