The AltSoftSerial claims it can do 57600 baud.The SoftwareSerial claims it can do up to 115200 baud.But has a major disadvantage: If using multiple software serial ports, only one can receive data at a time. http://arduino.cc/en/Reference/SoftwareSerialFor a single serial port, use the SoftwareSerial. It is a well working library.What is it for ? Do you need a higher baudrate than the common 9600 baud ?With higher baudrates, the other device must have a precise clock with a X-tal.
Does this also apply for SoftwareSerial?
If you don't mind, another question is whether the baud rate chosen for the SoftwareSerial independent of the baud rate for hardware UART.