nextprime or isprime - which is faster

[zak seidov <zakseidov@yahoo.com>]

i run (elementary) pari for 12-13 digit numbers looking for some primes,
and wonder which command is faster nextprime or isprime.
I use: 
 
Reading GPRC: /cygdrive/c/Program Files/PARI/.gprc ...Done.
                  GP/PARI CALCULATOR Version 2.3.4 (released)
           i686 running cygwin (ix86/GMP-4.2.1 kernel) 32-bit version
 compiled: Jul 12 2008, gcc-3.4.4 (cygming special, gdc 0.12, using dmd 0.125)
                (readline v5.2 enabled, extended help available)
                     Copyright (C) 2000-2006 The PARI Group
 
thanks
zak seidov
 
btw is "extended help available", i guess not