I've owned and extensively ridden both small and medium-framed Santanas, as I'm basically right in between the two sizes. I ended up selling my medium tandems and bought smalls due to the dropped top-tube Santana uses on most of its newer small tandems. (I had back surgery and wanted the lower standover height of the drop tube design.) You really need to try both frames and see which fits the best. Remember that stems are easy to swap out to get longer or shorter reach. Despite being 5'8" I have a short inseam (29-30"), which makes bike fit challenging. Standover-wise I should ride a small; cockpit fit-wise a medium is better.
Assuming the small fits, and that both are used bikes in otherwise similar condition, if it were me I'd save the nearly $2k and get the Sovereign. The only real difference in spec I see is it's missing a carbon fork, which you can easily add. (I have both a carbon and steel fork for my main tandem, as for touring I swap out the carbon and use the steel fork.) Apart from the lateral tube on the Sovereign, they are similar framesets (Scandium).
And finally, I'd say the Sovereign is worth more like $2000-2200 than $2600. I'd offer $2000 to start.