the 427 seems to me to have been almost an afterthought to the 407 instead of being designed specifically to meet market demand. slapping two engines onto what is essentially a 407 airframe musn't have been what most twin operators were looking for, hence the 429 with a redesigned airframe. the 427 reminds me of the 206LT, in that its a proven airframe with a f'n ugly engine compartment thrown on top...
this is all just my speculation, i don't really have experience with either of the twins, maybe someone else can be more insightful...