Really simple. Get on board and see how the regular service works for you. If you are within a few hundred miles of land likely regular will be fine. If on a TA/TP cruise than probably need high speed and at times that will be VERY slow. Same for cruises to Antarctica and Greenland.
Much of your download speed depends on how close you are to the router. Since not streaming videos etc, you could move to get the few documents you might need.
No need to choose now. Also you might do a search as there are several threads on exactly this recently.