A gel pencil goes on soft, stays movable for roughly the first minute, and then sets. That window is the whole product: it is what makes a smoked line possible, and it is what forgives a hand that is still learning. A liquid liner has no window at all -- it lays an opaque edge exactly where the tip went and dries as drawn. So this is not about which is better made. It is about which mistake you would rather have. A sharp graphic line with a steady hand wants the liquid; anything soft, or anything still being learned, wants the pencil that can be rescued.
Waterproof is a removal decision, not only a wear one
The property that keeps a line intact through a warm room is the same property that makes it refuse a normal cleanser. That is worth choosing deliberately rather than defaulting to the more durable option: an oil-based remover on the eye area every night is a step, and skipping it is how a waterproof liner ends up half-removed and sitting in the lash line.
People who own both use them on different days
These are not two versions of one product competing for a slot. The pencil is a soft eye and the liquid is a drawn one, and no single line is done well by both. If only one is being bought, buy for the look worn most often rather than the one aspired to: a wing attempted twice a year does not justify a tool that only does wings.