We’re gonna need a bit more detail about the robotic arm you want to build – how big is it? What material are you going to use? What do you mean by tight space, how tight?

Both the types of gearboxes you’re thinking of can work well, and both have been used in industrial robotic arms. You list a lot of things you want the gearbox to have (low backlash, even load distribution, durability, etc) – in my mind, the planetary gearbox wins in most of those categories. The helical-spur combo is more easily manufacturable, but a planetary gearbox is not that hard to manufacture, so I think it still ticks all your boxes.

For your compact robotic arm gearbox (15Nm, 25:1 reduction), planetary gears are superior for your requirements:

Low Backlash: 3-5 planet gears share load, achieving ≤5 arc-min backlash (vs. 10-15 arc-min in helical/spur combos);
Load Distribution: 150%+ higher torque density with balanced tooth contact pressure;
Compactness: Single-stage planetary achieves 25:1 in 40mm axial space (helical/spur needs 2 stages, 60mm+);
Shock Tolerance: Multi-tooth engagement handles intermittent loads better (FEA shows 30% lower peak stress vs. helical).
Critical Notes:

Use 20CrMnTi steel with case hardening (HRC58-62) for pitting resistance
Optimize carrier pin tolerance (H6/h5) and preloaded angular contact bearings
Avoid: Plastic gears – fatigue failure risk at 15Nm cyclic load
Example: Harmonic Drive®-style compact strain wave achieves <1 arc-min backlash, but planetary offers better cost-efficiency for prototyping.

I’m guessing this is pretty small, so the planet gears might be tricky to make depending on your material. So for me, in this particular situation, it’s the helical-spur combo.

But I agree, more detail on the application would help us to help you make a smart choice. And if low backlash really is an important factor in your decision, it’s gotta be the planetary. Planetary gearboxes are the most common in industrial robotic arms for that reason.