Normally, I wouldn't weigh in on these, since so much depends on the dice... but I am also a speed player, and prefer faster units.
5/8/5 is not enough of a speed advantage over 4/6 to make that much of a difference. For one thing, anytime those jump jets are used, you are adding a +3 mod to the Griffin's shooting.... and really, are pretty much only useful when you are already at a high enough heat that you cannot run to cover, to cool down... at which point, adding 3 to 5 heat points, to a unit that only has 10 heat sinks, and is already overheating enough to lose movement, is unlikely to be a lot of help.
In addition, the only time that the Griffin will be able to take advantage of the blind spot on the Goliath is going to be in turns where it is both close enough, already, to run to that location, as jumping, with mods and heat would hurt your weapon fire, and also on turns where you win initiative, or the Goliath just turns around. Regardless, you have to hope you get something good out of it, and not just hit arm or leg, and also hope you can get away next turn, or you are vulnerable to an assault mech at a dangerous range.
It's not impossible for the Griffin to win, but, assuming relatively equal skill levels on the part of the players, and relatively statistical die rolls, as in, the Goliath isn't always missing on 4+, while the Griffin never misses a 9+ ..... then the odds are that the heavier armor, and slightly heavier firepower will win.
Nahuris