Solidity – Understanding the Difference Between Gas Price and Gas Limit

What is the difference between now and block.timestamp?

Nothing. They're aliases. From the documentation:

now (uint): current block timestamp (alias for block.timestamp)

Update: (Thanks @ShaneFontaine)

Solidity 0.7.0 deprecated the now keyword.

For contracts ^0.7.0, you must use block.timestamp.

There's a huge difference between them

function createFoo (address _contractAddress){
    Contract A = A(_contractAddress);
    A.foo();
}

If foo() fails the call to createFoo() will also fail and it will propagate reversing any change you have made.

Another difference is you can retrieve the value returned by foo(). If the function returned an address you can assign to a variable.

address who = A.foo();

In the second case if foo() fails, call will return false. But it will not reverse changes you have made, you have to explicitely call revert().

function callFoo (address _contractAddress) {
    address newAddress = _contractAddress;
    newAddress.call(bytes4(sha3("foo()")));
}

And you do not have access to the value returned by foo(), call only return true if it executed successfully or false if it has failed.