We have a limited time in Portugal and after spending 4 nights in Lisbon we wanted to explore either the Algarve or Porto...both have amazing sights but we can only choose one...PLEASE HELP!

It will really depend on your preferences and time of the year.
Algarve will be better in summer when u can go to beaches.

Porto it's very beautiful everytime of the year. Depending on the time you have, you could go to Serra do Geres, that is also amazing.

If you go to algarve with a renting car, i would suggest you to take the coastal road and do some stops along the way, there are beautiful beaches there too.

I like both but the Algarve is mainly tourism and golf courses with the odd real town scattered in and in many ways the region north of the Algarve (the alentejo just over the main east west road) is more the real Portugal.

Porto is a working city with some very pretty destinations in it and near to it. Porto is very much a wine town and if that is of no interest then you may end up a little bored.

Algarve can be very touristy yes, but it really depends. There are beautiful places to be with no crowds.

But if u want to stay away from crowds, i agree with the previous answer, and i would suggest maybe alentejo coast, and destinations like: arrifana, monte clerigo (one of my favorite beaches), odeceixe, zambujeira do mar. All this places belong to the Costa Vicentina. Keep in mind that the water here will be a bit colder than in algarve, bigger waves and more wind.

In Algarve, you could try some of the islands near lagos, culatra and farol or maybe tavira island.