I'm sure the GX model is due to it having a rear window spoiler.
SR's tend to have alloys, spotlights, sports interior & rev counter.
All the 1.3's have basically the same 75 BHP engine as standard.
Best way to know if you're getting a good Micra is by it having an up to date service history & a HPi check. Just a general look around to see if it's rotten inside the wheel arches, sills & front crossmember. Test driving lets you know if the gearbox is OK & if it pulls in any direction when driving, look at the drive shafts, track rod ends & suspension.