I have used SurveyMonkey in the past for many years. It's way cheaper than Delighted.com, has an API for integration with everything including Marketo, and can be used for more than NPS surveys, for all sorts of questionnaire-based research. Use the 75 euro/month option to get access to the API. It's 3 times cheaper than the 224 usd/month charged by Delighted.com
I chose Python for Data Science (Machine Learning, Data Analysis and Data Visualisation) and all the rest of the things it's able to do (practically anything) and because it's able to be used on its own for the whole pipeline and not just in an isolated environment.