This depends on your platform and your decision about the GNU linker. For most platforms, you'll need to use GAS if you use the GNU linker. For some platforms, you have no choice; check the gcc installation notes to see whether you must use GAS. But you can usually use the vendor's assembler if you don't use the GNU linker.
The GNU assembler assembles faster than many native assemblers; however, on many platforms it cannot support the local debugging format.
If you want to build shared libraries from gcc/g++ output and you are on a Sun, you must not use GNU as, as it cannot do position-independent code correctly yet. Note: I've been told that this problem has been fixed in the most recent binutils release, but I haven't checked this yet.
On HPUX or IRIX, you must use GAS (and configure gcc with the
--with-gnu-as option) to debug your programs. GAS is
strongly recommended particularly on the HP platform because of
limitations in the HP assembler.
The GAS distribution has recently been merged with the binutils distribution, so the GNU assembler and linker are now together in this package (as of binutils version 2.5.1).