I am also deciding between the Explorer II (black dial) and the Sub Date. Here are a few points I have thought about (in no particular order);
You should be able to get the Exp II at least 20% cheaper than the Sub although their list prices are very similar. Purely because the Exp. II is not as popular.
If you are planning on selling the watch in the future, the Sub will hold its value better although this could be offset by buying the Exp II at a discount.
The Sub seems a common watch; the Exp II is slightly different which could make it more appealing to you.
The Sub let's everybody know you have a Rolex the Exp II is much more discrete.
Functionality - do you require the rotating bezel or is a 24hr hand more useful?
I believe the Exp II is more comfortable than the Sub, but I can't back this up.
I still can't make up my mind - only one way...keep trying both of them on until you think I can wear this watch for the next 10+ years.
Hope this helps
Anuj