Do bear in mind that not all smartphones from Sprint will be able to support the new Sprint Spark service which improves on the existing 4G LTE service, as one would need a newer device in order to take advantage of this benefit. Spark also offers another added bonus which would be HD Voice support, which happens to be a wideband audio technology which theoretically ought to improve voice quality.
Consumer Reports engineers have confirmed that Spark is unable to function like the older Sprint 4G service which could maintain cellular data in the midst of a phone conversation. I suppose if multitasking is a huge deal to you, then it might be best to avoid Sprint Spark for the moment, although there are non-Spark 4G LTE phones that do not have this issue.