Lessons in Vocabulary: Ostensibly and Presumably

Obstensibly
Ostensibly:

Etymology

From ostensible < French ostensible < Latin ostensus, past participle of ostendō ‘I show’, from ob ‘before’ + tendō ‘I stretch out’.

Pronunciation

* (RP) IPA: /ɒsˈtɛnsɪbli/ or /ɒsˈtɛnsəbli/
* (US) IPA: /ɑːsˈtɛnsəbli/

Adverb

ostensibly (not comparable)

1. seemingly, apparently, on the surface

A bedroom is ostensibly the safest room in the house, but many parents started to think differently and install better window-security features when Elizabeth Smart was kidnapped from her own bedroom one night.

Synonyms

* (seemingly): apparently, arguably, at first blush, seemingly

-From Wiktionary

Presumably:

pre·sum·a·ble (pr-zm-bl)
adj.
That can be presumed or taken for granted; reasonable as a supposition: presumable causes of the disaster.
pre·suma·bly adv.

presumably
Adverb
one supposes or guesses; probably: he emerged from what was presumably the kitchen carrying a tray

ThesaurusLegend: Synonyms Related Words Antonyms
Adv. 1. presumably - by reasonable assumption; "presumably, he missed the train"
presumptively

presumably
adverb IT WOULD SEEM, probably, likely, apparently, most likely, seemingly, doubtless, on the face of it, in all probability, in all likelihood, doubtlessly

-From The Free Dictionary

Subtle difference... yes. In a nut shell it boils down to: on the surface (ostensibly) vs. taken for granted (presumably), but they can have essentially the same meaning. Even some of the Thesaurus words match: seemingly and apparently. In case you use one instead of the other, you're probably not misusing the word itself. You will, however, give a different shade of meaning to your sentence than you were ostensibly presumably going for.