JASJAR and K-JAM both have the same ROM and RAM capacity. The JASJAR runs on an Intel XScale processor at 520MHz, a single-core CPU. The K-JAM runs on a TI OMAP 850 processor at 195MHz, a multi-core CPU. It has been proven (and based on firsthand experience) that the K-JAM performs on par with a 400MHz XScale CPU on many tasks, since the processor's focus is on power, not speed.
There is a moderate size difference, too. Some will say it's not much, while others will say it's a big difference. Those who are very much on-the-go will appreciate the smaller screen and lighter weight (160g) of the K-JAM, versus the larger screen and heavier weight (280g?).
Lastly, depending on your region of the world, you might appreciate the JASJAR's UMTS technology and 900MHz support, both of which are not supported in the United States. If you live in the US, the K-JAM's 850MHz support and EDGE technology make it a great unit to have.
Did I mention that the JASJAR is big?