Not Portugal specifically. I've toured single-o a bit, with generally positive experiences; I expect the same general rules of thumb would be appropriate for Portugal, if perhaps a little more relaxed - rumor holds the roads are excellent.
I'd always recommend self-guided touring. The idea of a guided tour seems a bit untenable - any assemblage of riders will have quite some variety in skill and capability, and staying together is bound to be a strain on almost everyone.
Touring bike, absolutely. The flexibility inherent in carrying your own gear is vital. My trips have been on road bikes, and 95% on roads. MTBing does appeal - it strikes me as the land-based equivalent of a canoe trip, with notions for the day instead of goals, loads of diversions, usually sleeping out...I've talked me into it. Next time, off road.
So, what are your plans? Just wanting to vacation with your bike? Can't beat it.