On the oxyacids of +III pnictogens and +IV chalcogens Phosphorus in its $\mathrm{+III}$ oxidation state is known to exist as the phosphonic acid tautomer rather than phosphorous acid one. All salts isolated containing $\ce{H2PO3-}$ are (to the best of my knowledge) hydrogen phosphonates rather than dihydrogen phosphites. Similarly, all $\ce{HPO3^2-}$ salts should be phophonates rather than hydrogen phosphate. Only as organic esters (which cannot tautomerise easily as carbocations are less stable) are both forms observed: trimethyl phosphite and dimethyl methylphosphonate both exist.

A similar picture is true for sulfur in the $\mathrm{+IV}$ state. Sulfurous acid and sulphonic acid are both known as far as I know, and both structures have been isolated in salts. Similarly, methyl methylsulfonate and dimethyl sulfite are both known to the best of my knowledge.

The corresponding elements of the second period do not show this tautomerism. For oxygen, this is due to its high negativity preventing the formation of ‘oxyacids’ altogether. For nitrogen, this is because an $\ce{N=O}$ double bond is more stable than two $\ce{N-O}$ single bonds so that nitrite ($\ce{NO2-}$) is already devoid of protons and cannot tautomerise. However, remnants of this behaviour still exists in organic compounds where both the nitro group $\ce{-NO2}$ and organic nitrites $\ce{-ONO}$ are known.

What is the case for the heavier homologues of both phosphorus and sulfur? Do the elements arsenic/antimony/bismuth and selenium/tellurium/polonium exhibit both an -onic acid and an -ous acid or is one form preferred over the other? Is there a general trend going down the groups or are secondary effects (e.g. lanthanide contraction) more important?

Answers backed with references to crystal structures of anions (or the free acids) and/or corresponding quantum-mechanical calculations (both in references or self-performed) are preferred.

Note that the fully oxidised pnictogen(V) or chalcogen(VI) compounds are not part of this question,
Disclaimer: This isn't really the answer Jan is looking for. 



Holleman, Wiberg "Lehrbuch der Anorganischen Chemie", de Gruyter, Vol. 101, notes that:

Selenium 

Selenium forms a weaker, but more stable acid based on oxidation state $ +4$. No explicit mention of the structure, which suggests to me that $\ce{SeO(OH)2}$ is the structure.

Tellurium

Tellurous acid's structure is described as unknown.

Arsenic

For arsenous acid (the $+3$ oxidation state based acid) the tautomeric equilibrium is entirely on the side of $\ce{As(OH)3}$ (as opposed to $\ce{HAsO(OH)2}$). The reason given is that $\ce{As}$ does not form double bonds with $\ce{O}$, which is outdated: Current thinking is that only the first row forms double bonds, i.e. no double bonds in $\ce{H2SO4}$ or $\ce{HPO(OH)2}$. Organic esters exist that can be described as $\ce{RAsO(OH)2}$.

Antimony

The $+3$ oxidation state based acid is described as forming $\ce{H+ + [Sb(OH)4]-}$ in water. No mention of tautomerism.

The book gives sources only sparingly and on a per-section basis. Relevant items appear to be:


R. Paetzold: "Neuere Untersuchungen an Selen-Sauerstoff-Verbindungen", Fortschr. Chem. Forsch. 5 (1966) 590-630.
W.A. Dutton, W.Ch. Cooper: "The Oxides and Oxyacids of Tellurium", Chem. Rev. 66 (1966) 657-675.
M.A. Ansari, J.M. McConnachie: "Tellurometalates", Acc. Chem. Res. 26 (1993) 574-578.
J.D. Smith: "Arsenic, Antimony and Bismuth", Comprehensive Inorg. Chem. 2 (1973) 547-683.
C.A. McAuliffe: "Arsenic, Antimony and Bismuth", Comprehensive Coord. Chem. 3 (1987) 237-298. 
GMELIN: "Arsenic", Syst.-Nr. 17, up to now 1 book, ULLMANN, Vol. 5: "Arsenic and Arsenic Compounds", A3 (1985) 113-141.
GMELIN: "Antimony", Syst.-Nr. 18, up to now 6 books, ULLMANN, Vol. 5: "Antimony and Antimony Compounds", A3 (1985) 55-76.




My thoughts on $\ce{As, Sb}$ are that in the MO picture, $\ce{H}$ has insufficient overlap to form a bond that would compete with the alternatives. 



I have performed DFT calculations (PW6B95-D3/def2-QZVP//PBE-D3/def2-TZVP) on gas-phase $\ce{Pn(OH)3}$ and $\ce{HPnO(OH)2}$. Of course, these are gas-phase  energies on single conformers, not free enthalpies. For $\ce{Pn} = \ce{N}$, a 10 kcal/mol favorable energy difference towards $\ce{HPO(OH)2}$ was found. For $\ce{As}$, about 35 kcal/mol towards $\ce{As(OH)3}$ was found. The Mayer bond order analysis was not conclusive.