You should use :utc_datetime unless you have a reason not to. The reason why Ecto’s timestamps() are naive by default is backwards compatibility. Since they are always UTC anyway, you should use :utc_datetime for them (and I believe the default will be changed to that in the future).
The difference between :utc_datetime and :naive_datetime is that the former will ensure that you can only insert UTC DateTimes and will read back UTC DateTime structs from the DB, whereas the latter will remove timezone information when writing and return NaiveDateTimes when you read from the DB.
For most cases, you should use UTC.
BTW, if you need to store the timezone too (in PostgreSQL), you have no option but to use a naive datetime field and store the offset or timezone information separately. This is because PostgreSQL does not have a data type for storing a timestamp with timezone.
There is the confusingly named timestamp with timezone but all it does is convert your input timestamp to UTC and convert it back to whatever your DB connection’s timezone is when reading (so for most cases it’s useless). It does not even store the offset/timezone. So you need to do that yourself somehow.
