Buckle strap knee pads are the most secure option out there, using plastic or metal buckles to lock the pad firmly in place. They're the go-to choice for race skiing and high-impact freeskiers who can't afford any pad migration during a crash. If you've ever had a knee pad slide out of position right when you needed it, you'll appreciate the locked-in confidence buckles provide.
The trade-off is that buckle systems are bulkier and harder to adjust on the fly compared to Velcro or slip-on styles. You won't be fine-tuning the fit mid-run, so getting the initial adjustment right at the start of the day matters. They also tend to be less comfortable for all-day casual skiing.
Choose buckle straps if you're racing, hitting big features in the park, or skiing aggressively where impact protection is non-negotiable. For resort cruising or milder days on the hill, a Velcro strap or hybrid system will likely feel more comfortable while still staying put.