New phone and service provider advice needed please

[Towser]

I am now in the market for a new phone and also a new service provider, any recommendations Guys and Girls? I currently have a Samsung S3 and have been looking at the S5 and Iphone 5s but would like to hear from anyone who has either and what they think.

I am in the UK so only UK based providers required and NOT EE or any company associated with EE! I was looking at GiffGaff and O2 but I heard GiffGaff users get restricted by O2 as it runs on the O2 network and O2 seem to give preference to those on O2 over those on GiffGaff.

Thanks in advance

Ron