The answer is, 'it depends', partly on what network your phone is on, what phone it is and what you expect. Unless your phone is 4G and your dongle is 3G you won't see any speed difference, you will, however only need the one contract so you could ditch the dongle. If it's an iPhone you can only tether using the main four networks so GiffGaff, Tesco and Virgin won't allow it, not sure about Android but I think most would work, I know for sure that GiffGaff won't allow people with unlimited data to tether though.