I don't think there is much likelihood of the GT-51 coming out anytime soon. Locosys made a few prototype GT-35's with both a 5 hz MTK chip and a Standard SirF 3 chip but they didn't like the economics enough to put it into production.
I have one of the protos and I can say it does not seem to have any significant advantage over the GT-31 for most normal uses.
A new case design would be nice if it could be more waterproof but I don't think anything is on the drawing boards there either.
There have been rumors of other brands building something specifically orientated towards GPS-SS but nothing has come of it yet. I think it would take a year or two even if they are full steam ahead and then it would take a while to test and asses it for accuracy and reliability.
I think you are pretty safe to get a new GT-31.