/@/ or /I/?

This topic was up in a different thread and I thought it was interesting so I'll make an individual thread for this topic.
So when do you use /I/ and /@/?

Many dictionaries say that suffix -ment is pronounced with /@/, but around here the majority pronounce it with /I/, so government is pronounced /gVv@`nmInt/. Also "admit", "accept" and "advice" all have /I/.

However, object(verb) has /@/ and never has /I/.

Any rules as to when to use /I/ and /@/?

When an /I/ is in an unstressed position, it can optionally be pronounced as /@/.
It is just a matter of style or preference. In faster colloquial speech one is more apt to realise /I/ as /@/ in the positions mentioned above.

e.g.: opportunity - both "i"-s can either be pronounced as an /I/ or as a schwa /@/.

>>It is just a matter of style or preference. In faster colloquial speech one is more apt to realise /I/ as /@/ in the positions mentioned above.<<

Depends on the dialect, as in many North American English dialects [I] is the preferred weak vowel in most positions rather than [@].