You always can test them both and compare the results. It's not that hard. The first one should be faster.
And by the way, the 2 queries can return different row... You do not guarantee in any way that with the second you will retrieve the record with id = some_int
They would be the same speed unless a field in the where clause is indexed and reduced the cardinality. (The number of records to check). In that case, the first query would be faster. If the field in the where clause is NOT indexed and MySQL needs to perform a table scan the second query would be faster. Also just to clarify, by same speed I mean the difference wouldn't make a difference, performance wise.