Baron wrote:

The difference is negligible. They both suffer from unbalanced feed
currents and thus feeder radiation. There is or was a commercial
design that attempted to reduce that.

73's

Modeling of J-poles and ground plane verticals shows considerable mutual
coupling to the feedline, and the Slim Jim almost certainly behaves the
same in this regard. The amount of common mode current this induces and
consequently the amount of pattern distortion it causes depends on the
feedline length and its path to ground. When the current is significant,
you'll probably need two current baluns (common mode chokes), one at the
feed point and the other about a quarter wavelength down, to reduce it
to a low value.

I theorize that the dependence of common mode current on feed line
length is a possible cause for the widely varying reports on how well a
J-pole radiates -- some people get lucky, some don't.

Roy Lewallen, W7EL