Arantxa was extremely talented (anyone who wins a Slam at age 17 defeating Graf is).
She is just more skilled than Amanda. I like Amanda, but a commentator once said that Amanda is a 'B Grade' version of Arantxa.
Jenn, don't get me wrong, but you don't really seem to know much about Arantxa (gamewise). She hits the ball pretty hard (not Serena style but hey, its pretty hard).
One thing Arantxa worked on was her serve and it is much more 'beefier' now than it ever was.
Arantxa knows how to use the net well, she was a world No. 1 in doubles for a good few years and has something like 60+ doubles titles.
Why is Arantxa better than Amanda (and most of the tour IMO), she had variety in her game, wasn't always a counterpuncher, was more aggressive, as Saki said, had good court sense and her mental toughness was unquestionable.
Plus she was also 1 in singles and doubles, made 12 GS finals. The above paragraph summarises why Arantxa was so good.
She wasn't just a human backboard