Well you'll get different opinions on this. But here is mine.
I don't use either. I've never used phosphate remover, mainly because i've never had a problem with phosphate, that i've needed to resort to putting iron in my tank. You should be careful putting iron in your tank, It causes a lot of interesting things to happen.
I use to use carbon occasionally, but have stopped completely now. I use UV instead, it's much more effective in my opinion ( http://www.fnzas.org.nz/fishroom/uv-why ... 10335.html ). Carbon is a source of phosphate, even the ones which specify they are low phosphate, still contain phosphate, they just weren't washed in phosphoric acid like it usually is.
BUT if you are going to use phosphate remover, then I recommend running carbon straight after it. It helps to reduce some of the not so desirable effects of iron based P removers.
If you keep your tank clean, and have a decent skimmer, there is no reason why you need to run either carbon or phosphate remover.
Layton