This is a very subjective question, but can sometimes be answered by looking at the differences between the two devices.
Fundamentally the Cosmo is designed to be used as both a phone and a handheld computer. While the Gemini has phone functionality it’s nowhere near as easy to use. If you want to use the device as a phone, get a Cosmo.
If you’re only after a device for Linux use then a Gemini will probably be a safer bet. You won’t need the telephony features, and at full price the Gemini is around $200 cheaper than the Cosmo.
The things that can disrupt this however are features that are exclusive to the Cosmo, such as its backlit keyboard. If this would be helpful to you, then a Cosmo it must be. Equally, if you’re prefer more memory and more storage, then that too would make you choose the Cosmo.