Here's a brief list off the top of my head, I'm sure there's more but it's a start.
Jammin runs smaller wheels and tires out of the box. The ofna runs the standard 2.2/3.0
jammin runs a smaller body to go with the smaller tires, so if you want to run standard tires you need to cut the wheelwells.
Ofna body is ugly IMO, jammin is probably the most scale out there, I personally think their more scale than the associated body, but it's smaller
the jammin can run proline protrac wheels and 2.2/3.0 tires and still fit under the body (once the wheel wells are trimmed) giving it a narrower stance. In some peoples opinions that makes it more nimble.
Ofna uses a pillowball front suspension, jammin is triangulated, however the jammin can be converted to pillowball.
Strc makes hopups for the ofna. Xtremerc makes carbon fiber for both
With the proper shock tower, the jammin can run rear shocks up front for more dampening and travel
both can run the same motor and be happy
jmin has side mud guards, this will help protect the servo, ofna is open to the world.
Battery tray is more larger battery friendly on ofna, but that's an easy fix
front bumper seems to be the weak point on both
overall you really can't go wrong with either one, the jammin is always the mire race oriented side of ofna, but the choice is yours, hope I helped