JOINs vs simple multiple references?

[mmonaco]

What is the advantage of using a join such as

SELECT users.firstName, users.lastName, locations.city FROM users JOIN locations ON users.id=locations.user_id;

vs

SELECT u.firstname, u.lastName, l.city FROM users u, locations l WHERE u.id=l.user_id

I find the latter far easier to read, however is there a major efficiency advantage to JOINS?

[mmonaco]

I read that a comma separated list as demonstrated in my second example is the equivalent to an Inner join. In such a case can an ON argument be included or is it necessary to replace its function using the WHERE clause?