Mega<>Pro Mini, via serial or I2C?

I am looking at using my Pro Mini as a single-function module, a flow-meter, in conjunction with the Mega, and it will sit directly over the end of it. This means the I2C pins are as close to hand as serial3, and there may be some value in using I2C. What are the pros and cons of using I2C compared to serial? I won't be short of serial ports.

I2C is very limited in length. I've had troubles with cables as short as 6 inches. With some effort you can push it out to 20ft.

20ft should be no problem at all for regular Arduino Serial. If you want to go further or you have some local interference then RS232 adapters boost the signal voltages up to a level that's much stronger.

The Arduino libraries for I2C don't support the full protocol, so if you were thinking of using some more advanced features like broadcast addresses, then you have to write your own.

Serial also has limited functionality but in practise, that doesn't hurt 99.9% of what you want to do.

Thanks. Since the ProMini shield will sit on Mega, the distance will be just a few mm. I have just realised that, since Windows is not involved, I can bump up serial to 230400 or more,so, unless I2C has a speed advantage over that, I guess this is not worth pursuing. I'm not looking at anything fancy, just a number from one source.