I take it you already decided against the seatpost, If you have a rack too, any bag fitted on top would likely obscure it.
I'd have thought the rack would be the best place.
Firstly it's a bit higher than the mudguard, so might be marginally more in the line of sight of a dozy car driver who's more interested in his phone call than driving.
Also, it'll save putting holes in the mudguard. We have a couple of cheapie solos with little cateye lights on the chromplastic type mudguards, the lights get knocked occasionally and the fixing holes get damaged and the plastic starts to split. I've had to refix mine at least once because of this. I think a light on the rack will probably be more secure and protected a bit by the rack.
Our tandem is fitted with the Thorn carrier with two Vista nebula's, and they fit very well and securely. I think the Thorn rack might cost more than a Tubus one, but I expect you could still fit the lights in the same way.
