Is there anything specific the JK has that you want, or just want a newer vehicle? The down payment on a new one could probably add whatever you want to your TJ.
Assuming you are making a decent down payment to keep the payments under control, and don't intend to finance for 72 months or something crazy like that.
I've seen several people with TJs post that they don't really care for a JK after test-driving. So, try that first. If you like it and still want to upgrade, pop into the JK forum and ask about customizations you're interested in over there. Someone has probably done it.