Your first code probably does not initialize correctly for a two line display so you are operating at (1/8) duty cycle which results in the contrast being readable.

The second code initializes correctly for a two line display operating at (1/16) duty cycle, which is why is less readable or dimmer. This is why you need a variable resistor for the contrast.

With the 16MHz you will need to increase the number of loops in the delay routines, unless you use a timer.

Use the flow chart I posted, make sure the delay times are met, and put a variable resistor on the contrast. Format and comment your code.

Jon