I've only done decoction with German beers...never Porter. Sounds fun!
As to extract/all-grain, I have yet to be convinced that you can make as good beer with extract as grain. And it costs as much as buying beer if you use extract.
I can always tell extract brews. They're almost always sweet. There seems to be something in the extraction process that carmelizes sugars and makes more unfermentable sweetness.
Now, I haven't made an extract batch in probably 15 years, so maybe extracts have gotten better, but a single-infusion all-grain batch, once you have the equipment, is a piece of cake, cheaper ingredients, and, in my opinion, better quality. You do have to have the space for equipment, though, and I wouldn't bother with 5 gallon batches for all-grain.
Also, I just enjoy the process of going from grain to beer. But, hey, to each his own. Just keep on brewing!