Which is more secure?

Hi,

I've been pouring over the Exim documentation and source code for
almost a week now and am still confused about one thing...

They're pretty strict about security where I work, so I was trying to
determine which is more secure: setuid or setuid+seteuid.

Now, from some of the articles about security I've been reading, seteuid
is sometimes considered not as secure as setuid.

But, the way Exim is written, does Exim stay in "root" mode longer
in "Setuid" mode than in "setuid+seteuid" mode? That is, if we were trying
to reduce the amount of time the code is running with a uid of root,
would you choose "setuid" or "setuid+seteuid" security mode?

P.S. We are running on Solaris 2.5.1 on Sparc Ultra

Thanks!