Hi there,
The best option for large amounts is the bank.
It depends on the islands after that. If you're visiting very touristy islands like Sal or Boa Vista, you can pay almost everywhere in euros. On less touristy or more mountainous islands like Santo Antão or São Nicolau, it's better to have escudos.
Either way, if you pay in euros, you'll lose out on the exchange rate, which is usually 1 euro for 100 escudos instead of 110. So it's better to exchange money.
Marie
Faites que le rêve dévore votre vie afin que la vie ne dévore pas votre rêve.