Alex, you really should help us with a sketch.
If you use I2C you need a Master + X number of slaves.
You can achieve a similar thing with Serial using 2 wires (1 wire if you are really clever). Plus ground of course.
BUT, you MUST have a little think about the Slaves comms back to the Master.
A standard USART serial out is a pin in
'output' mode.
And, also, non-USART pins sending Seriout will be in
output mode.
So, ALL of the pins can drive the line high or low and will be fairly low impedance.
Question.
What happens if you connect a bunch of Serout 'output' lines together?
Once you have worked out the potential problem FOR YOURSELF , then we can give you the answer and the solution.
No doubt some smart ass will tell you the answer but it would be REALLY GOOD if you could work this out for yourself.
I find that if I can suss the problem then the solution follows it fairly quickly.
Which method is best? I2C if you can make it work. Either will be fine. Make your mind up
Blimey , if you are running out of ideas what to build now , then this school holiday is going to be pretty boring. I suggest going for a game of tennis or football with your friends. It'll give you a chance to think and burn off the Christmas Turky flab