Hi. I’m not living for almost 2 years now in Portugal, but in my time the ISIC card was not very popular. The best card to have was the EURO<26 card. That one was accepted in many small shops even, and I think also on buses. But not inside cities, only long distance buses have discounts. The ISIC may be useful however in some museums in Lisbon.
So my suggestion: if you can, make both ISICand EURO<26 :)