Utrecht is very easy to reach from Schiphol Airport.
There are direct trains every 15 minutes from the airport to Utrecht Central Station. From there it is a 5-10 minute walk to both conference hotels.
Please check the railway website for more details: http://www.ns.nl/en (one-way, 2nd class, €8.70)
More public transport information (including door-to-door navigation with walking directions and all forms of transport) can be found at: https://9292.nl/en
If you prefer to take a taxi from Schiphol, it is best to book a taxi in advance (€55, one-way) at the Utrechtse Taxi Centrale: http://www.utc.nl/english/index_english.html
If you don’t book in advance, taxis from the airport may easily add up to €100.
Additional information on how to reach the venue can be found on https://ise2a.uu.nl/information/venue/