Arduino to arduino.. what to choose

Hi.
I`m thinking what protocol to choose.

I have 2 boards away from each other around 5m(inside the car)
now, on one end is display and some buttons, in the boot 5 sensors and 8 logic transistors.

what would be the best to choose??
I allready used SPI for display, so I2C or UART?

thanks for any info.

UART, but use proper RS232 converter chips. The 5v TTL lines won't work well in the automotive environment. The proper chips give +/-12v on the lines, so they are more resistant to noise and other nasties. I2C is not supposed to go more than 10cm but you can add booster chips that will take it up to 20m or so.

CAN bus would be good but that's a little more hardware at each end.

USe modbus protocol ; make master & slave configuration

MorganS
Can you please explain what`s the booster chip.(with example) are we talking about repeters??
the whole intention is to use as less cables as possible hence I2C is ideal for me with properly shielded cable.

AMPS-N
"USe modbus protocol ; make master & slave configuration"
Is this can be implemented within arduino?? what about the cable lenght. is there any support/example for them??

This is my first attempt to get 2 devices talking together.

Use shielded cable and be sure to ground the shield.
CAT-5E cable is available at many hardware stores.

http://playground.arduino.cc/Code/ModbusMaster

ya support is avilable.but you should know how modbus protocol works.

There various library developed here, used it well.

goForSmoke, printer usb has shield around the wires, should i connect the shield to the USB ground??

I have just found that:
P82B715
http://www.nxp.com/documents/data_sheet/P82B715.pdf

This should solve the problem for I2C distance.... i guess.

mOskit:
goForSmoke, printer usb has shield around the wires, should i connect the shield to the USB ground??

What is printer USB?

There is one ground on an Arduino board that all the ground pins and holes and the USB connect to.
They are all the same.

With shielded CAT cable you have a chance to use straight serial between the boards. The hardest part would be soldering to the thin wires but Scotchlok connectors can solve that or since there are 8 wires in the cable you could twist 2 pairs together for each line and use the shield for ground.

You need cable anyway whether serial or I2C. You can test it for the either way, simplest first.

If you need more voltage on the cable then you can circuit that and give it external power. Advantage is that you can use serial code you should already know without extra libraries.

i only need to send signal and +5V supply to the display and touch.
What would be more sufficient Serial or I2C?
If i use serial TX RX can those pins be used as digital input/output as well??
If i use I2c SDA SCL can those pins be used as digital input/output as well??

If you wire RX and TX then you are using those pins for digital I/O.
Same with SDA and SCL.

How do you plan on powering your project? I would run a separate power wire and maybe ground, speaker wire could carry those.

I get the feeling that you don't know much about electricity and hardware.

I`m completely lack on all of this stuff, got some knowledge but not enough.
Learning something new everyday.