My advice is to mix it up. Here are some suggestions below:
1. Slow negatives (on the way down): Fight hard and fire those hamstrings for 5-10 seconds for every negative. Once at the bottom either explosively push up or reset yourself by having your partner or holding device release pressure. Keep the reps relatively low (4-8) so long as your TUT for the eccentric remains constant or close to. Experiment with the amount of sets and expect much soreness.
2. Slow positives (on the way up): Lower quickly under control and use the least amount of resistance to raise up. Again fire the hamstrings and try to come up slowly, usually 3-5 seconds. Reps can be high for these type of sets (10 or more).
3. Bottom-ups: Start at the bottom (lying down) and have your partner say “go” or you mentally say “go” and fire those hamstrings explosively, with minimal hand pushing raise up. With a partner this type of change can give dramatic results. The partner should provide no pressure until he says “go”. This provides tremendous shock to the muscles and since you are emphasizing the positive portion there is less damage. Reps should be mid-range (6-12). My personal favorite is to use bottom-ups for low rep, multiple sets.
Glute-ham raises can be performed quite often and should be if time permits. Work up to doing them 3-4 times/week employing the different types listed above. Here is an example of doing Glute Ham Raises 3 times weekly:
Monday (leg day): 4 sets GHR of 4 reps using slow negatives, 1 set of slow positives of 12 reps.
Wednesday (non-leg day): 4 sets GHR of 10 reps using bottom-ups.
Friday (leg day): Same as Monday.
This is just an example and there are many ways you can train GHR’s. Let me know if I forgot to address thing or if you have more questions.