No view on Peach, never met him, although a number of people I respect sing his praises. I have however, met Adm Z and rate him highly.
If there is RN angst at the appointment of Peach, I would suggest it probably stems from a combination of the following :
1. It's 13 years since a naval service officer held the post of CDS. That is three terms of the Army and a very long term of RAF (Stirrup). There are some understandable reasons for the Army bias (given contemporary operations), but no longer. It's fair to say that there is a sense that it really should be the navy's turn - not in a strict rotational sense, but generally because the return to contingent ops ought to require that sort of perspective. It is also noteworthy that in the 30 years since Fieldhouse took up the post in 1985, only he and Boyce from the NS have actually held the CDS post. There are some who will say - with some justification - that this may not reflect well on those in the RN who achieve high rank.
2. Adm Z is very highly regarded and as a leader is a step change from some of his more recent predecessors. I suspect there is a palpable sense that if he couldn't make it, there'll be a long wait for the next one. This may or may not be true and it might not be a dark blue suit that becomes the next leading candidate, if the rumours are true.
I sincerely hope Peach lives up to his reputation and proves to be the best man for the job. I have no doubts that Adm Z would have proven equally capable and am sad to see that he'll be leaving in April.