First Class or Normal train ticket Lisbon-Lagos

There are two types of train from Lison to Tunes (where you have to change for Lagos) Alfa and InterCidades.

Alfas are tilting trains very similar to those used in Italy and Britain but not quite as fast. They have just been rebuilt. https://www.cp.pt/passageiros/en/discounts-benefits/Highlights/novo-alfa-pendular They are branded Conforta and Turistica but the tickets are sold as 1st and 2nd class. if you have a 1st class reservation you can pre-order a reasonable meal depending on time of day (sample menu https://www.cp.pt/StaticFiles/Passageiros/3_viajar/0_servicos/ap/menu_comboios_eng.pdf)

InterCidades are simpler with a cafe car, Ist class will get more space and will be quieter.

After Tunes the onward train is pretty simple and I'm not sure it has 1st class.

If you sign up with CP.pt and become a "client", then you get bargain prices. It is free.
1. Always sign in when planning your trip.
2. Buy your tickets between 8 and 30 days in advance.

The price differences, for your Lisboa-Lagos trip, are:
1. AP 1º and 2ª is €4 in promotional pricing.
2. The complete price range is €30 down to €11 depending on train/class/time-of-day.

For me, if the AP departure time is convenient, then I would travel AP 1º for the 2.5 hours for €19.