Public transport in the Netherlands is very good, trains running at least every 30 minutes between Rotterdam and Amsterdam during the day (every 15 minutes during rush hour) and hourly throughout the night. PT in the cities is really extensive as well. Taxi's are expensive and taxi drivers are not used to be hired for long rides, since the perfect PT system. More info and travel advice (in English) on www.ns.nl (site of the Dutch railways). For a complete free PT travel advice (from address to address) in English, use www.9292ov.nl, or call 0900-9292 upon arrival in the Netherlands.

Traveling PT with a lot of luggage might be less feasible, especially during rush hours. You might consider taking a taxi, but count on paying at least 80, more probably 100 euro for the trip, might be a bit cheaper when you try to book in advance, though. PT is usually a bit faster too, because the congested road around Rotterdam, Amsterdam and the couple of cities you'll be passing an-route...