they're both amazing figures. so, it really comes down to which movie you like more. then price.
as price goes, the P1 is right at the limit of what i'd pay for almost any 1:6 figure.
but, you actually get what you pay for. unlike the older pred figures which have skyrocketed on scalpBay.
the T-600 probably won't go too high like the T-800 has and there will probably be plenty of chances to get it in the future.
the P1 will probably get out of hand pretty soon so you should grab it now and get the T-600 later on.
or, you know... get both.