Both build length and structure. The difference is how they behave in the thirty seconds before they cure, and that difference decides which one suits how you work.

Builder gel self-levels

Place it and it settles into a smooth surface on its own. That is a real advantage: less filing afterwards, and a naturally even apex if you have placed the bead correctly. It also means gravity is working the whole time. Pause too long and the product travels toward the cuticle or the sidewall, and you are chasing it.

Builder rewards a technician who works quickly and confidently into the lamp.

Polygel does not move

It stays exactly where you put it until you cure it. There is no clock, so you can shape, step back, adjust, and shape again. The trade-off is that nothing self-levels - every contour you want, you create.

Polygel rewards patience, which is why it is the better first system for most people.

Structure is the same either way

Whichever you choose, the apex belongs over the stress area, roughly a third back from the free edge, and the sidewalls stay parallel. A thin apex in the wrong place fails in a fortnight regardless of which product built it.

The honest recommendation

Start with polygel. Move to builder once your shaping is fast enough that self-levelling helps rather than hurries you. Most technicians end up keeping both.