Is there any way to test which clustered index is better for specific queries?
The Clustered Index should be defined with a narrow and ever increasing value (this is why so many of them use an auto-incrementing integer).  

Having worked with large tables like this before, your best bet is to continue to use an auto-incrementing primary key as the definition for the clustered index.  Then create non-clustered indexes that support your queries, with the understanding that other ad-hoc queries will probably perform terribly.  You get the added benefit that the index on your large table should never need defragmentation. 

Given that, creating a non-clustered index on Timestamp, EventType should give sufficient performance.  You typically want the most specific column referenced first, and timestamps are good candidates for this, especially if you always (or almost always) include it in your query.  

The advantage of non-clustered indexes is of course that they are typically much narrower than their base table and so require much less memory.

CREATE NONCLUSTERED INDEX TimestampEventType
ON IndexTest1 (Timestamp, EventType)


You can also benefit from casting to the proper datatype (I'm assuming DATETIME)

SELECT * FROM IndexTest1
WHERE Timestamp &gt;= CAST('2018-03-14' AS DATETIME) AND Timestamp &lt;= CAST('2018-03-20' AS DATETIME) AND EventType = 'String'


Finally, check that statistics are up to date for your query and that the execution plan is actually doing an index seek or range scan with a key lookup back to the main table.