I doubt it's either.
If you're gonna start throwing parts at it you need to get Auto Enginuity.
Buy it here, best price, best support http://www.itpdiesel.com/store.php?crn=135&rn=426&action=show_detail
You'll need a windows mobile device or a laptop to run it. You could swipe a cheap laptop for this purpose only for a few hundred I'm sure.
then you need to learn how to use it.
You replaced glow plugs and relay but did you verify that they are all working properly still yet?
Have you load tested the batteries? It takes a minimum rpm for it to start. If the batteries are cranking it the least bit slow it will be a slow or no go.
These engines are rediculously power hungry at startup for lots of reasons.
Check fuel pressure.
Does it smoke or miss when it starts? What color?
Don't just throw parts at it. There is enough expertise in this forum (and I'm not talking about me) to fix this problem at night with their hands tied behind their back and blind folded.
Just stick around, ask questions, and approach things in the logical manner and we'll get er fixed for ya.
I will also consult my Ford service manual DVD and see what the troubleshooting process for hard/no cold start is in case we missed something.