The pronunciation of tremendous

There are two different versions of British pronunciation for the word "tremendous" on Oxford and Cambridge dictionaries respectively, which one is correct?

Oxford: /trəˈmendəs/
Cambridge: /trɪˈmen.dəs/

Answer

What is involved here is less the pronunciation than the dictionaries' different standards for representing the pronunciation.

The vowel here is ‘reduced’: that is, it is not only unstressed but largely deprived of any character which distinguishes it from similar vowels. For a long time all such reduced vowels were regarded as allophones of a single phoneme, conventionally represented with the schwa, /ə/; and that is to this day a very common dictionary representation, as in your Oxford citation.

In the ‘40s and ‘50s, however, it was recognized that in one context there is a meaningful contrast between different realizations of this ‘phoneme’—in final syllables. The parade minimal pair (two words or phrases which differ in only one phonological element) is the contrast between Rosa’s and roses, which almost all native speakers of English pronounce differently. The reduced vowel in Rosa is the mid central vowel represented by /ə/; that in roses is a distinctly higher, a ‘near-close central unrounded vowel’. According to Wikipedia:

In the British phonetic tradition, the latter vowel is represented with the symbol /ɪ/, and in the American tradition /ɨ/.

Wikipedia says that the OED and The Oxford Dictionary of Pronunciation for Current English have recently introduced yet another symbolization:

A symbolization convention recently introduced by Oxford University Press for some of their English dictionaries uses the non-IPA "compound" symbol [ᵻ] in words that may be pronounced with either [ɪ̈] or schwa. For example, the word noted may be represented [ˈnəʊtᵻd].

So your two variant notations represent the same range of pronunciations in different ways. In your own speech it probably doesn’t matter at all which you use; nobody is likely to notice, unless you exaggerate. After all, even professional phoneticians didn’t care much about it for a hundred years.