i am very new to the m17x and in fact selling it. will wait for others to chime in because i dont know if the r1 can use iX cpus. however between the 620 and 720, i wanted more cores so went with the 720. if you dont need the cores, the higher clock of the 620 may be better. it all depends on your use.
i run virtual machines, so the quad with hyperthreading was my choice, and with 8GB ram, it runs great.