I've been to both Sardinia (in October) and Lisbon (in May). Personally, I'd get bored in Sardinia for a month. The only large city is Cagliari in the south and although it's pleasant, there isn't as much to see there as in many mainland Italian cities. The beach towns would be sleepy in February. Though there are trains on Sardinia, you'd be somewhat limited without your own transportation. There's a lot more to see and do in Lisbon and in nearby towns. I had hot weather in Sardinia in October, and cool weather in Lisbon in May. February weather in Lisbon doesn't sound appealing.
Though I haven't been to Sicily yet, but I think it would be a better destination in February than Sardinia. Better February weather is found in Andalucia (southern Spain including Seville, Granada, & Cordoba, plus the nearby "white villages"). I had perfect weather in Morocco in February last year, which is easily reached by ferry from southern Spain.
In January-February this year, I spent 4 weeks in Guatemala, Belize, & Mexico. Cities like Oaxaca and Mexico City have lots to do and great February weather. Another idea is Buenos Aires, though it may be a bit too hot & humid in February.