Both very similar,
Max-stim is based on the same principles of HST only with fatigue management added to the mix. Fatigue mangement is the key principle of max-stim, in theory it allows two benefits...the reduced fatigue allows the muscles to recover from the exercise more quickly than conventional sets, and it definitely has been proven to allow the muscle to complete more work with a given load in less time...i.e.- if 100 kg is your 5 rm for bench press, you could try to do 4 sets of 5, but this would be exceedingly difficult with conventional sets due to the metabolic fatigue of straight reps...or you could do 1 set of 20 reps with an adequate m-time between the reps to allow you to complete all 20 reps consecutively.
On the flip side, since HST uses conventional sets and metabolic fatigue, IF and I repeat IF it turns out that metabolic fatigue and stength endurance are proven to be effective stimulators of hypertrophy, then conventional sets would be better, but as to date, there is no evidence that this is true.
Which one is more effective? Impossible to say at this time...both HST and max-stim are so similar, that they probably have similar effects on hypertrophy. Max-stim in theory should be slightly more efficient, but this is not proven.