difference of pronunciation

I'd like to know if there's a difference in the pronunciation in the second syllable of the two following words according to RP pronunciation: briTAIN and wriTTEN . I thought there were two possible ways of pronouncing, either with vowel n°12 (shwa) or not but my teacher keeps saying that they are pronunced differently. I wait for your answer, thanks!

[iconoclast]

The second syllable is pronounced exactly the same in both words, with unstressed schwa (or syllabic /n/).