They are two different phones.  
The 'prime' is officially called the S5 LTE A. It's Got a 2K display,  the Snapdragon 805 chip and is compatible with LTE catagory 6. The phone is not going to be sold outside of South Korea. 
The Galaxy Alpha is a 4.7 inch device with a 720p display. The main focus of the device is the premium build quality (metal trim). It is not waterproof,  lacks a micro SD card slot and has a 12 mega pixel camera.  So far it has only been announced in select markets.
Posted from my S5