Let me see how much i can help.
For starters here is a website that can let you do good Spec comparisons between 2 or more devices (Already got the GP5 and Walkman Z comparison set up for you): http://www.theverge.com/products/compare/2564/3687
Now for SQ, I don't own either of the devices to be honest (Well i do own the GP5s newest younger brother, the GP 4.2) so i can only tell you what i have heard about them.
The Sound Quality for the Walkman Z i have heard is comparable to that of the Cowon J3, a popular PMP among Head-Fiers here that was discontinued a few months ago by the company and has very good SQ and battery life. It also has a MicroSD Slot like all the Galaxy Players for expanding your memory, but from what i heard it has a maximum file limit of i recall 8000 files (If anyone can correct me on this if i am wrong that would be appreciated) that you can store on it internally and externally.
As you can see on the website above the Walkman Z has a far less support of audio codecs than the GP5 and has no MicroSD support so your going to have to pay more money for more space. As with most devices these days like the Ipod Touch some of the space is already used for the OS, and i heard that half the 8GB Sony Walkman Zs space is used for the OS so if you go this route you will probably want to get 16 GB or more. For FLAC playback, you could buy a music player from the app store that supports FLAC to play it and battery life as far as i know is not good with most Sony devices. I am unaware of the DAC used in the Walkman Z also.
For the Galaxy Player 5, as you can see supports alot of audio codecs and is capable of having expanded memory through the MicroSD Slot (They say the maximum is 32 GB only when expanding memory but i have seen plenty of people use 64GB SDXC cards with this device and other different devices after formatting it to FAT32). Sound quality i have heard is good and is even better for the GP 4.0 and GP 5.0 when you root it (Like jailbreaking an IPod) and adding Voodoo Sound on it.
From what i know battery life is better on here since it is a bigger device than the 3.6, 4.0, or 4.2 as it uses a 2500mAH battery, if you use it for just music like i try to do with my GP 4.2 then it will last you quite a while unless you start playing games or watching videos of course. Only downfall with the battery on the GP5? Unlike all of its brothers it has no removable battery. This device uses a Yamaha DAC, which i hear lots of people say is not as good as a Wolfson DAC (I do not know as much as other Head-fiers on DAC chips), the GP 4.0 and 4.2 use a Wolfson DAC and i do not know about the GP 3.6.
As with all devices like the Galaxy Player Series and this Walkman Z series if you want to keep the battery alive for a long while before the next charge it is recommended to turn off all wireless signals the device is putting out (Ex. Wifi, Bluetooth) if you are not using it.
Hopefully i have managed to help you. If anyone can fix or add more to my information please do so.