I think your room is very nice, but I'd want to dial it up a little, too. I'm not a fan of turquoise mixed with your current scheme, but I think you could add a softer, simpler fabric sofa, still in terra cotta, and generous in size, and replace the leather chair with one upholstered in a bold black and white print.

Then a couple of lumbar pillows on your tan/gold chairs, also in black and white, and a pair or quartet of big prints with large pale mats and simple frames over the sofa, and I think you'd get the boost of energy you need without disrupting the total scheme in your house.

Here's a link that shows many rooms with sofas. You could go gray, dark gray, brown tones or patterns, a navy, cream, there are so many choices but this is good as you get to see them in rooms that are decorated.

Here is a link that might be useful: couches

In planning a color scheme start with what will stay. If you plan to keep the SW Blonde, the yellow chairs, and carpet that is the basis of your scheme.

IMO your room could benefit from more pattern which could be in the drapes, upholstery, cushions, or art.

You ask some difficult questions which, really, only you can answer. I recommend looking at a lot of pictures and save the ones which have something you really like. Gradually you will find the things you really love and know what you must add to your decor. However, I'll give some opinions on your questions which possibly *may* help.

I'm looking for opinions on whether to replace them (sofa and chair) with fabric or leather.

This completely depends on personal preference, lifestyle, and perhaps to a degree on budget.

I also have 2 fabric chairs in the room that I'd like to keep, but not sure the color works.

IMO the color of the gold chairs seem to work well with the SW Blonde walls especially if they are not directly in front of the walls.

Would it be okay if the new couch and chair are a different style from the 2 current chairs?

Yes, they are transitional style so will work with several styles.

I have no idea what color to get. (for the sofa)

I recommend checking out print fabrics either online or IRL. These are designed by professionals who know which colors look best together. Focus on prints with gold and you may see colors you like that will work well with the gold in your room. Otherwise you might go with a dark brown, dark gray, or as BM suggested, terracotta. I'm not suggesting a print for the sofa altho that *could* work but solid colors on large pieces generally are easier to decorate around.

I once saw a butter yellow leather sofa that I loved, but too much yellow?

Not necessarily too much yellow if you like a monochromatic scheme altho it may not work with the wall and you won't know that until it's in place. If you really want this color sofa be prepared to repaint.

So, how does one decide on a fabric vs. leather couch...and what color?

Already asked and answered. :)

I'm thinking the new side chair should be a print fabric, right

Yes, IMO the room needs more pattern and that is a good place to provide it. You need not be limited to one pattern only - when you look at fabrics you will see there are many companion fabrics with different designs using the same colorways. For example, a large floral print, a smaller geometric or floral print, and a stripe using the same colors.

Bronwynsmom...thank you for the ideas. I wish I had the guts to do your black and white suggestion. it conjures up a very stylish image. I never thought of a terra cotta sofa, but will look for some pics to see how that might look with my bright yellow gold chairs.
Yayagal, thank you for the link. I've spent hours on Houzz, it is helpful to see whole rooms.
Luckygal...I really appreciate the time you took to answer all my questions! You've given me some focus. Funny, but I stopped by JoAnne Fabrics on the way home from work tonite just to gaze at fabrics and see what colors comb's work with gold. Problem is I love almost everything! But looking at fabrics is an excellent suggestion.
I found this I like...what do you think of something like this (sofa and foreground chair)?

Here is a link that might be useful:

I think your choice is beautiful. I have two sofas in that same tone and they're so versatile in that I can put so many different treatments on them with pillows etc. depending on the time of the year. They'd work very well with what you have already.

Actually, you could sneak up on the black and white idea with something traditional, and a creamy rather than stark white background.

You could go buy a couple of half-yards and throw them about to test the idea, and perhaps you won't need extra courage!