Benchmark two separate SELECTs versus one LEFT JOIN

Has anyone seen a benchmark on the speed difference between:

SELECT * FROM item WHERE id=123;
and
SELECT * FROM vendor WHERE id=515;

versus:

SELECT * FROM item LEFT JOIN vendor ON item.vendor_id=vendor.id WHERE
item.id=123;



I only have a laptop here so I can't really benchmark properly.
I'm hoping maybe someone else has, or just knows which would be faster
under high traffic/quantity.

Thanks!