Two ways to hold a screen in the air
A monitor arm has one hard job: carry the weight of your screen out over the desk and then hold it, wherever you leave it, without creep or sag. There are two common ways of doing that, and the listing usually names which one you are getting. A gas-spring arm balances the load with a sealed cylinder of pressurised gas, the same idea as the strut that holds up a car tailgate; the monitor-mount resource Monitor Arm Guide describes the part as "a sealed gas cylinder (similar to a car hatchback strut)". A mechanical arm, often sold as spring-assisted or constant-force, uses an internal coiled steel spring instead, tensioned once at setup. Both end up in the same place, a screen that floats and stays put, so the real difference is in how they get there and how they behave over the years rather than in what they let you do on the first afternoon.
How each one feels to move
Lift a gas arm and the screen glides. The pressurised cylinder pushes back with much the same force across the whole travel, so the monitor rises, drops and tilts under a light, even touch; Atdec, which makes mounts, says its gas arms "enable one-finger positioning for most monitor sizes". A mechanical spring can feel a shade firmer in the hand, with a little more to work against, though plenty of people like exactly that. Atdec calls it "positive positioning feedback that many users prefer", and a stiffer action is less inclined to drift once you have set it. The sharper split shows up the day you change monitor. A mechanical arm carries a tension screw you turn with an Allen key, so you can wind the spring tighter or looser to match a heavier or lighter screen later on. A gas arm is balanced by its cylinder and offers no such adjustment, so it needs matching to the screen from the outset.
Weight is the number that actually decides it
For a single, ordinary home-office monitor, either mechanism will carry it without complaint, and this is where most buyers can stop fretting over the label. The one figure that does matter is weight. Every arm, gas or spring, prints a load range, and a screen under the bottom of that range drifts upward on its own while one over the top sinks however you set it. So before the mechanism, settle the weight: read your monitor's figure off its box or spec sheet, and hold it against the range on the exact arm in front of you. To put real numbers on how far those ranges differ, Atdec rates one of its gas models for roughly a 6 to 16 kg screen and one of its spring arms for a lower 3 to 10 kg. Heavier panels, and most two-screen setups, lean towards gas for that headroom; our best monitor arm guide prints the weight range beside every pick, and the dual monitor arm guide does the same for two screens on one mount. The bracket pattern on the back of the monitor is a separate question again, and our VESA fit guide walks through that one.
Which lasts, and which costs more
Longevity is the clearest win for the mechanical side. A steel spring is a plain part with little to go wrong, and because you can re-tension it, an arm that begins to droop after a few years can often be wound back up rather than thrown out. A gas cylinder ages differently. Monitor Arm Guide puts it plainly: "the sealed cylinder gradually loses force over the years, and it cannot be re-tensioned to make up the loss", and Atdec agrees that gas systems "may require replacement sooner due to pressure loss". That gap is part of why gas tends to cost more at the till, a premium Atdec places at around a fifth to a third over a comparable mechanical arm. None of which crowns one mechanism for everybody. A light screen that lives in one spot and rarely moves is well served by a mechanical arm, the cheaper and longer-lived of the two. A heavy screen, or a desk where you shift the monitor between sitting and standing all day, is where the effortless gas glide earns back its premium. Decide the weight and the habit first, and the mechanism follows from them.
FAQ
Q: Is a gas spring or a mechanical spring monitor arm better? A: Neither wins outright; they suit different screens and different habits. A gas arm moves more smoothly and carries heavier monitors, which is why it turns up on dual-screen and sit-stand desks. A mechanical arm is firmer to move, usually cheaper, and tends to last longer because you can re-tension its spring instead of replacing a tired cylinder. Settle on your monitor's weight and how often you expect to reposition it, and the better mechanism for you falls out of those two answers.
Q: How can I tell which type an arm uses before I buy? A: Start with the words in the listing. Gas models are sold as "gas spring" or "pneumatic"; mechanical ones as "spring-assisted", "mechanical spring" or "constant force". The surer tell is how the arm is balanced: a mechanical one has a small tension screw, usually set with an Allen key, while a gas arm has a fixed, factory-set balance and no such screw. Monitor Arm Guide warns that "gas spring" is sometimes used loosely for any counterbalanced arm, so the tension screw, or the absence of one, is the detail worth trusting over the headline word.
Q: My monitor arm has started to sag. Can I fix it? A: That depends on the mechanism. A mechanical arm nearly always has a tension screw, so a sag usually means the spring wants winding back up with the Allen key the maker supplies, a job of a minute or two set out in the manual. A gas arm that sags has typically lost pressure in its sealed cylinder, and since that cannot be re-tensioned, the fix is a new arm rather than an adjustment. Look in your manual for a tension screw first; whether one exists tells you which of the two situations you are in.