on the picaxe any output pin can be used as a serial output and any input can be used as a serial input is this not the case with arduino? My devices do not support i2c. The picaxe only has 27 bytes of variable space so any serial strings longer than that would be lost. So the scratchpad can be used as ram I guess you would call it. Basically I need the arduino to accept a data string filter that information and write it to a sd card.
The picaxe does serial in software which puts extra load on the processor. The Arduino can do this too with the softwareserial library http://www.arduino.cc/en/Reference/SoftwareSerial
The Arduino also has a hardware USART which handles all all the bitbanging independently of the processor, so you don't need to worry about delaying and much faster speeds are possible. But the USART is only available on certain pins.
I2C is something else.
If the scratchpad is ram, the Arduino has 1024 bytes. This is memory you can use to store variables which is lost when the device loses power. There is also 512 bytes of EEPROM which can be used to store data and is kept when power is lost.