I would say agassis problem was his lack of footspeed. he did move well but lacked athleticsm and top end speed. sampras had the edge there.
If agassi was as fast and athletic as say djokovic sampras would have had a really tough time given andres quality of return and from the baseline. but agassi simply wasn't agile and fast enough even if he was a hard worker and very fit when he was on.