I cannot speak directly to the quality of either of the two versions you are looking at, but hypothetically if the fit and finish were the same, I would probably get the 21J. I have a A7750 3717 and I frankly never use the stopwatch function. If I am going to time something I use one of my gens - and seriously, I hardly ever time anything.

So, if all other things are pretty much equal (except the chrono function), go faux, and use the left over $150 to get another rep.