It's separate from your 3G service.
Maybe better, maybe worst but the data speed will sure be faster.
Remember you don't make calls on LTE, just data for now.
So much also depends on where you are. Inside of some 'dense' buildings, there can be a degradation of signal strength. And AT&T and Verizon are each implementing LTE in different ways, so it's kind of hard to give a 'one size fits all' answer.