Melding 101 crit will always increase your critical rate by 7%, while melding 101 speed will always reduce your GCD by 0.096.

Not considering procs, you would get 3.5% more DPS by melding crit.
Assuming you have a GCD of 2.4, you would get 4.175% more damage on your GCD abillities by melding speed.
Assuming you have a GCD of 2.3, you would get 4.364% more damage on your GCD abillities by melding speed.

The effects of critstats stay linear, while the ones on speedstats will get slightly better the more you stack them.
Speed has larger effects (usually more than 20%, which is alot) than crit but don't work on AA and dots. Speed leads to more TP consumption.
The mentioned 4.~% for speed are in practice also diminished, due to animationlocks when using offGCD abillities between GCD abillities.

Speed has also something like 'breakingpoints'. When you stack enough, you can fit one more action into timeframes of buffs.
That could allow the usage of superior rotations.