I'm not going to get into the pointless debate on which keeper is better as they are so different in style and indeed stage of their careers, however I have had personal experience of the coaching of both Achterberg and Valero, albeit his philosophy through other keeper coaches, through the time my lad was at Kirkby. I will qualify these comments by saying this was over 3 years ago so things may have changed.
Achterberg is a really nice fella who is obsessed with coaching, he used to be at either Melwood or the academy from early morning until 8 at night before he got the 1st team role. He was however a far more basic coach than those under Valero, his drills were more physical and at that time more old school. There was still an emphasis on footwork but nowhere near as much as under Valero's regime, or indeed Billy Stewart and Tim Dittmer before him at the academy.
I have said it many times before, I believe Valero was a far more intelligent coach with a philosophy on goalkeeping as good as any I have encountered at a number of academies. Maybe Achterberg's style of coaching is more suited to adults, I would imagine it is, however I could see an obvious difference in what both were about.
I know many on here will just generalise and say that keeping is just diving and keeping the ball out of the goal, but it really isn't and as I have found out over the last 8 years is one of the most complex positions in any team, mentally, physically, technically and tactically.