Without anyone knowing your actual application load patterns, they are completely guessing.
"MySQL server" can mean many things when you load profile a machine. We have customers that slam CPU, and barely touch disk since tables are loaded into RAM. We have other customers who don't touch CPU, but are maxing out 6 drive RAID10 arrays of 15k SAS.
It all depends on your load patterns. I would say your programmer (hopefully) will have more insight into this than some random hosting company sales rep, or a bunch of people on a message board.
If you want to talk generalities, go with faster disk every time. I would make an educated guess that 95% of our dedicated servers sit at 10% CPU usage or less, while disk subsystems are regularly overwhelmed. CPU simply doesn't matter for *most* hosting applications (there are of course exceptions to every rule).
So, if you trust your programmer go with what he says. If you don't trust him, get a different one
Good luck to ya.
-Phil