I like the avatar for your account :)
I think this really strongly depends on your threat model. People may disagree but pound-for-pound Qubes is definitely the desktop OS that is most difficult to compromise, the devs have very impressively designed an OS which is resilient to an array of attacks. Qubes is also very difficult for a lot of people to use (much more so than Graphene) and can be a pain to get working depending on what hardware you are dealing with.
It's very difficult to assess what is "better" for a desktop environment without knowing what sorts of threats you're worried about. Qubes frees your hand to, say, have some disposable environments routed over tor and others routed over a VPN or whatever (you can ofc do the same in Graphene using orbot, but orbot is a little rough around the edges...). Similarly Qubes allows you to keep some sensitive materials (passwords databases, sensitive documents, etc) in offline virtual environments (effective software airgaps) which can be powerful in protecting sensitive materials, something else difficult to emulate in Graphene. So if you're singularly worried about passive/dragnet surveillance there are benefits to using Qubes over Graphene in desktop mode, but those benefits may not be significant enough to warrant all the work in switching. Alternately, if you're worried about targeted surveillance/active attacks on your devices I'd say out the box Qubes is probably more resilient to an attack than Graphene simply because virtualization can be really effective at containing a compromise. The huge benefit to Graphene is the fact that with the auditor you can be virtually certain that the device has not been compromised in a persistent way, whereas I'm not aware of similar hardware-based attestation being possible, say, on a device running Qubes. So you may be more likely to be compromised on Graphene if you're being targeted, but you'll also have an easier time knowing that you were compromised on Graphene. In other words, there are upsides and downsides to both. I'd say if you have any activity which powerful adversaries want to get their hands on, it is worth considering having a Qubes system where you also run Tails for some activities on a desktop environment, just because eventually your Graphene devices becomes a single point of failure otherwise. But that totally depends on your threat model.