What''s the difference and which is better?

[Kinger]

...between an AGS 0 (ideal cut) and a H&A ideal cut?

 

[JohnQuixote]

Hearts & Arrows is a pattern appearing in a diamond when the cutting is so symmetrically precise that the facets align to create a kaleidoscopic effect when viewed through a special scope (H&A viewer).

See this link for info.

Note that even a diamond with non-ideal proportions could be cut to display the H&A effect...But the best performing H&A diamonds will be cut with ideal proportions.

Which is better:

Given two well-cut stones with equal, proven major proportions - one H&A and one non-H&A - the difference in overall performance will be slight.  However, the character of a well-cut diamond’s beauty (the visual balance of brilliance, fire, contrast and scintillation under a broad range of lighting) depends upon seldom-discussed minor facet construction.  An advantage of well-made H&A diamonds is that particular attention goes into the fine-tuning of these minor facets.

Buying a well-made H&A diamond is often seen as a safe way of assuring the diamond is cut to proven ideal proportions.  Since H&A facets are precisely aligned to maximize light return a well-made H&A diamond should be an optimum performer. 

Of course one can find non-H&A diamonds cut to equally proven proportions with maximum light return.

So, as with preference of color, clarity and size... The H&A choice is a personal one.

 

[strmrdr]

On top of what John said there is another consideration:
Hearts--Arrows-Diamonds---Its-not-all-about-light-return.aspx