Assuming I buy the new 5k iMac, is there a big difference between a 256GB SSD or a fusion drive in performance? I have a 2010 iMac now with a HDD. Whilst the 2010 iMac is crazy fast, the HDD makes it painfully slow. This turns me off of getting a fusion drive. Would the 256gb SSD be much faster? I could just get an external HD if I need storage.