I had a similar situation and UA proactively offered a $800 ETC which I took. The CSR email actually offered to send me the EU261 paperwork instead. I asked via the customer service form on the website. As a Silver, I don't know if the offer will be high enough, but $150 is too little IMO considering you have grounds to make a 600 euro claim for cancelled flight.
Technically I believe you can still pursue the EU 261 claim even with the customer service "gesture", but you can be assured they will fight it more.
There is a longer thread on EU 261 claims somewhere.