Serial 3 vs Software Serial
There are huge differences. The most important ones are:


SoftwareSerial is half duplex (it can send or it can receive, it cannot do both at once)
SoftwareSerial is blocking - while it is sending or receiving nothing else can happen


SoftwareSerial is very much a poor-man's serial port. It should only really be used for very low volume data (things like NEMA messages once a second or so) and only if you have absolutely no choice (and, let's face it, you always have a choice).