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I'm very biased, but I would go with Porto. It's larger, has cheap Ryanair flights to a lot of european cities and is more of a true portuguese city, not as touristy as Lagos.

Maybe Lagos is more touristy as summer holidays goes, but Lagos is as true portuguese city as Porto.

Reducing Lagos to the beaches and sun is nonsense.

Actually Lagos was one of the main cities of the country during the discoveries, even the main city of the Algarve.

ejinkayleekim, you  might get a bad perspective during the summer, when thousands of tourists "invade" Algarve, but if "practicing Portuguese" is your "top priority" you can practice in both!

So you're comparing two different realities: Porto now is the second metropolitan area of the country, and the weather is different from Lagos, and it's a much larger city than Lagos too. Lagos is a medium-small city in Portuguese terms (City since the XVI century) though its population increases a lot during the summer.

You should understand that "Portuguese culture" of the Algarve is also different from Porto, very much like that each region of Portugal has its own particular culture. For instance, there are some phonetic differences in the Portuguese spoken in  Porto or in Lagos, in the north they change the "v" sound with the "b" sound (so that "vinho" - "wine" - sounds like "binho"     and so on).

So this is all a matter of taste. Do you prefer the sunny south or the more rainy north? I'm sure you'll get a different kind of experiences in both areas.

I would say Oporto.  I live in Lagos and it is very English speaking, very touristy and not as friendly as northern Portugal.  I adore Lagos, but I am from south Florida and need the ocean like I need air.  My first three years were spent in a little village and hour north of Lisbon, I found it much friendlier and you had to fala to get by.  Also was able to enjoy great things like stray sheep in the village placa and the trucks with the fresh fish or bread coming right to your door every day. 

Yep, If you want to fala, go to Porto.
Bom Sorte!

Aha, Thank you all for really useful replies
It's really helpful for making a decision!