datetime vs timestamp in mysql 5.5
Like @a_horse_with_no_name told on his comment, the differences are documented in here, but here is some information:

Size:

datetime - uses 8 bytes for each field

timestamp - uses 4 bytes for each field (half of the size)

Range:

datetime - 1000-01-01 00:00:00 to 9999-12-31 23:59:59

timestamp - 1970-01-01 00:00:01 UTC to 2038-01-19 03:14:07 UTC

Timezone:

as @ypercube mention, timestamp converts your data to utc and store it, and when you retrieve it, it converts from utc to your timezone connection.

Concept:

datetime - Is a calendar date(same point in time can be different depends on timezone).

timestamp - Is a point in time, does not matter the timezone your are.

Suggestion:

The 2 main differences are range and size, then think, do you really need dates bigger then 2038-01-19 03:14:07 at the moment(at the moment, no in the future!)?
If no, go with timestamp for now, when you reach a point where you need a date range outside timestamp range, convert it to datetime.