I have only used APL but I understand J is a newer version that removes the need for the APL font which was always a bane of any installation. Yet, it seems strange for this to happen in the days of Unicode and graphics displays. I don't think it makes it any easier to read though.
[There are many versions of J, the early versions are freely distributable in source, http://www.math.uwaterloo.ca/~ljdickey/#apl-j is a good resource. More recent versions are freely available only in binary form under a non-commercial license, http://www.jsoftware.com is the resource here. One thing which distinguishes J from other languages in the APL family is that Ken Iverson -- the guy who invented APL (to make it possible to document the IBM 360 architecture, back in the early 1960s) -- is also the architect behind J.]
Also of possible interest is a writeup on A [L1 ] -- the implementation of A was important in getting J off the ground. Unfortunately, that page is a bit hard to read, because it expects that you have a special font installed with implied special character mappings -- they should have used gif images to represent those characters. Also, it's a bit old -- while that page claims that A+ "is is quite definitely not for sale", A+ is now distributed under the terms of the GPL.