I think it depends largely on what type of sonics you are seeking. I've read pro reviewers indicate that the Backert (particularly the Extreme) sounds close to solid state - very quick and handling transient well. This may be what you're seeking, while the Zesto may appeal to those seeking more of a classic tube sound.
Anyone compare Zesto with Backert preamps?
I am looking for a tube preamp and starting to narrow down my short list for auditioning. I know there are many wonderful preamps out there in the $10k and under category but I am particularly interested in learning about how these two well respected lines in particular compare to one another (doesn't have to be a comparison of their flagship models). Thanks for any thoughts you may have.