Definitely train. Rotterdam Centraal station to Amsterdam Centraal station takes 63 minutes and costs E22.50 for a return trip; I guess a taxi would cost closer to E100 one-way and will not be much quicker.
Unfortunately your hotel is not in the centre of Rotterdam, it's actually in one of the suburbs named Capelle aan de IJssel,
www.ov9292.nl (public transport journey planner) calculates best time as 1h 40m including walk, bus & train (from R'dam Alexander station). My advice is cab to Rotterdam Centraal (say E20) then train. Intercities run every 30 minutes at :00 and :30 from platform 9. Buy your ticket before you board. Return trains run through the night but only hourly and the journney is longer as they stop more often.
All in all could be a very long night. Hope you don't have to be in working condition the next day.