The difference is that in the one case, the activity has merit and is worthwhile in an educational sense-- both appropriate and even, I'd argue, necessary for the child in terms of holistic academic development.
In other words, EPGY, etc. aren't specifically targeted methods of producing higher test scores.
I think that most parents here would NOT view the complete Barron's series of SATII prep books as a good "plan for afterschooling" at least so far as it goes. KWIM?
Yes, enrichment probably does lead to higher test scores as well-- but it also leads to HG+ children who are more fully engaged, more fulfilled, learning to embrace challenge, etc. Shouldn't the focus be on learning in an appropriate environment? Why the focus on "scoring well" which is precisely what those test prep booklets are targeted toward? Have you examined one of those lately? They are NOT about "mastering the challenge" by a long shot. No. They are about "how to game this particular system" in order to look better than you really are.
That's not a message that I think that most perfectionistic gifties need to hear-- they are already prone to imposter syndrome.
I guess I have pretty firm opinions on this subject, being in the middle of having those test scores REALLY, REALLY matter with a HS senior-- believe me, that comes soon enough. No need to put that on a 5th or 6th grader's plate. The entire industry is suspect, IMO-- and for evidence, just see "SAT alarm" thread for details about that.
What IS true is that if you're bright enough and well-educated enough, these things really will still take care of themselves. Like I said-- a few practice tests just to get a feel for the level of stamina needed and the format. That's enough.