the ball race makes the servo output stay wobble free for longer, i used them on my Diamond dust where any slop will = flutter and you crash and die.
but... then you get into 148 vs 3003 debate, the 3003 is newer, has smaller electronic components and is better in most respects, its also cheaper than the 148. Because the 148 still has some processes done by hand apparently.
after that you get more and more powerfull servos, these are ones you use when you go bigger or faster