Directv HD for boat

I seached the forums but I cant seem to get a good answer.

Im looking to purchase a Satellite system for my boat that picks up Directv. I cant decide if I need the Intellian S6HD or the D4. They both say that the pickup the KA & KU satellites. The s6HD is almost $3K more then the s6HD.

If anyone has some insite for me I love to hear.

Thanks in advance.
Chris