My vote goes to an internal SSD as well. 
I have a Z440 with a Z Turbo and boot the computer from that for only one reason: I need all the SATA ports inside the Z440 to internal drives. If the Z440 had more internal SATA ports i would not have bought the Z Turbo to free up one SATA port.
Get the Kingston SNA-DC2/35 and place the 2.5 inch SSD drive in. The Kingston SNA-DC2/35 fits the 3.5 inch drive caddy in the Z620: www.newegg.com/Product/Product.aspx?Item=N82E16817991012