X-heads vs J-heads: what's the difference??

Between the two heads IF so EQUALLY equipped, there going to flow & run nearly dead identical. The problem with the 1.88 J is once you open it up for a 2.02, the bowl is small.
Equal porting done on the heads will yield similar results.

Someone mentioned splitting hairs.

I so agree.

Someone mentioned run what ya hot.

Again I agree

If you do not have either head, run the cheaper of the two. Or the one that fits the intended application over cheaper.