The biggest difference is that K63 and K64 are part of the new second generation Kinetis K devices (K2). Beyond what Mark and Justin posted, one of the other new features is crystal-less USB capabilities to save on BOM costs. Also there are also power and cost savings available with K2 parts depending on the devices selected.
There's also a ton of new enablement rolling out alongside these K2 devices:
The bootloader and Kinetis Development Studio (KDS) already support some KL devices currently. Other devices will be added further down the line. The new K2 devices will be first along with other newly released Kinetis parts, and then over time select older Kinetis devices will be added as well.
That's a great question! I would recommend checking out the K6x family fact sheet to get a good perspective on these families. Also, expanding on what Mark provided, here's a high level summary of the product families:
K60 :High Mixed Signal Integration
K61 : High Mixed Signal Integration w/ tamper detection
