We assume that Daemonic = More powerful, but does it? Might Daemonic just = Differently powerful? Now, I'm not saying this is the case mind, but just throwing it out there.
What would daemonancy do for a Primarch? Do we have real written examples of them becoming vastly more powerful immediately, or does it take time? Will their new form give them the ability to engage fully with the warp, or just with the things that meshed with their attitudes beforehand? EG, As a Daemon of Khorne, will Angron just be able to power himself with the immortal anger of Khorne himself, or could he make the ground turn to water and turn bystanders into spawn as well? We do know that they are never truly killed, just banished to the warp for a pre-determined length of time, so there is that.
In the end, is a daemon Primarch better in every way, or would Angron just be more, well, Angron-y? In there, somewhere, I feel might lie some way of making a choice that isn't just based on what character you happen to like most.
For me, getting off my tangent and striding manfully back OT, I think it would be close, but a clear head might prevail over boiling fury and mindlessness. So Sanguinius for me.