Tube rolling preamp or power amp gain tubes, which ones (or both)?

Both can greatly affect your presentation, depending on how much cleaner or warmer the new tubes are, compared to the stock ones. Nice thing about your components is you can swap 12AU7s between them, to gauge the differences. The effects will be cumulative, as you change tubes along the signal path. Try a MATCHED pair of NOS Mullard(Blackburn) GZ34 rectifiers, for openers. (https://www.youtube.com/watch?v=1FJ31Id3GTQ)  These are also phenomenal tubes, for the power supply: (https://www.upscaleaudio.com/collections/vacuum-tubes/products/philips-5r4gys-made-in-holland)

Always start in the preamp and work your way downstream.

How this works is: if you don't get the signal right upstream, you can't fix it downstream. Have fun- don't drive yourself crazy (and by that I mean don't obsess over it- you can ruin tube sockets that way...).

My experience with the PL3, PL5, PL7, PL8 matches the information outlined in the above post by rodmann99999.  The 1st stage or input tubes making the most difference opposed to a driver stage, but don't discount the rectifier tubes in the Preamp, they also can make a huge difference, most notably in the bass tone, lead and trailing edges. Also I have found that upgraded fuses improve the sound quality. Because of the total  of 12 signal path tubes  between the pre and the amp , I assume you are trying to determine which of the two components will provide you with the most improvement so you can replace a fewer amount of tubes then 12, at least for starters. Upscale Audio would probably be able to advise you best. Another approach would be to purchase two tubes, and install them in either one of the units (input stage). Allow them to break in for 200 hours, then evaluate them for a few sessions in said chosen component, after which swap, them to the 2nd component and listen for a few sessions. The above referenced process should allow you to determine the given tube/component combination with the greatest overall improvement to SQ.  I believe that the input tube stage signal tube is the most inboard tubes (left and right),
but I am not 100% confident in stating that as a fact, so I suggest you contact Upscale Audio for that info, or seek the info out in a forum here on AG.  Also I would adhere to the advice posted by atmasphear with regard to limiting tube replacement to keep wear of the socket pins at a minimal; although I would say that since  PL mounts there sockets on metal framed sub-chassis opposed to a circuit board that the PL is probably a lot more durable since your not imposing stress to a circuit board when changing tubes. 
Would it be possible for you to post what speakers, speaker wire, and IC's you have chosen to mate with your PL gear?  The question has less to do with tube selection and more with the personal interest I have with PL systems.
Brad

Contact Kevin Deal at Upscale Audio regarding your PrimaLuna tube upgrade choices. No one knows more about rolling tubes in PrimaLuna products than Kevin