Hi PK-
I have both and, while the share styling similarities, they are different animals.
Handle: The Gemini is noticeably smaller. It disappears better in the pocket, but sometimes I struggle to find the right grip. The Velox really melts into the hand, plus the hidden flipper lets you grip closer to the blade for great control.
Blade: Blade length is virtually the same. The Velox has a hint of recurve and is hollow ground from thicker stock. The Gemini is full flat ground and made from thinner stock. It's a better slicer (IMHO).
Flipping action: Here's where it gets fun. The Gemini snaps open. It's light and fast. Light switch the flipper and "POW" the blade is locked. The Velox has the smaller, hidden flipper. Combine that with a heavier detent and more weight in the blade and that thing explodes open. Push button the flipper and "KABOOM". I chuckled the first time it flew open.
Pocket clip: It's minor, but I thought I'd mention the differences in the clips. The clip on the Gemini is shorter, with more abrupt bends. The Velox's clip is more gently curved. I feel it more closely follows the style of the handle.
I hope that helped. If you're leaning toward the Velox, mine will be coming up for sale shortly