About ELF and COFF

[libra]

Hello,all

There are some object file format like a.out,COFF and ELF

The COFF is used by System V Release 3(SVR3) UNIX.
The ELF is used by System V Release 4(SVR4) UNIX.

What is different between these two object file format?
In other words, which one is better?

I think there are some reasons why the SVR4 use ELF,not use COFF as it's 
object file format.

                                                   thanks a lot!!