Table Design - Log Number Of Visits From Clients

I am creating a simple database to log the number of visits from clients. The table is as follows :

tbl_statistics

--------------------------------------------------
p_key | client_id | client_datetime
--------------------------------------------------
1______4________2013-12-05 21:00:00
2______3________2013-12-05 21:02:11
3______1________2013-12-05 21:07:31
4______3________2013-12-05 21:12:42
.....

It will probably be logging about 2-3 rows in this table per second maximum so there will be some rows ... My question to you experts now is how the design of this table is done in the best way ( performance wise) in terms of "primary keys" , " clustered index" , "non clustered index"...

The only type of query that will retrieve data from this table is a standard "SELECT FROM" where " client_id " is of a certain value and " DATE_TIME " is between a specific date range.

example:

SELECT Count ( client_id ) AS totantal FROM tbl_statistics
WHERE ( client_datetime ) Between '2013 - 01-01 00:00:00 ' And '2014 - 01-01 00:00:00 '

Or is this better?

SELECT Count ( p_key ) AS totantal FROM tbl_statistics
WHERE ( client_datetime ) Between '2013 - 01-01 00:00:00 ' And '2014 - 01-01 00:00:00 '