I'm compiling a table comparing the usage of prepositions 'in' and 'at' with the word 'university' using example sentences. Which preposition would be best suited to the following sentence:
- I'm in my third year in university.
- I'm in my third year at university.
Explanations accompanying your answer would be much appreciated, and if possible, a rule of thumb non-native speakers can follow when choosing between 'in' and 'at'.