Time dimension or timestamp in fact table?
I would recommend including both on the fact table. The dimension should be used for filtering and grouping, while the time stamp value can be used in detailed reports/queries.

Unless you care about whether an event occurred at 8 seconds or 42 seconds past the minute, create your time dimension at the grain of 1 minute.

As you did not tag your RDBMS, I thought it prudent to mention that later versions of SQL Server do not allow date + time data type operations; i.e. recreating the time stamp from the date &amp; time dimension business keys.
A work around is convert(datetime, [date column]) + convert(datetime, [time column]), or similar.