The main difference is that second class is 2-2 seating, while first class is 1-2 seating, so the seats are wider in first class and there are fewer people in each car. Many people in first class are business travelers who value the extra space to work, and often are not paying for their own tickets. On the other hand, if you're buying advance discount tickets, sometimes first class is only a bit more money than second class.
The Man In Seat 61 is a great resource for all train questions.
Here's his Portugal page. If you scroll down, you'll see some pictures of various Portuguese train interiors in both classes. http://seat61.com/Portugal.htm
Here's part of the Spain page; again, scroll down and look at the pictures of the various train interiors on the various routes, so you can see for yourself if first class is worth it to you. http://seat61.com/Spain.htm#London-to-Malaga-and-Seville
And congrats on joining the forum!