Either muffler choice will be okay. Probably a little better than stock but I wouldn't expect to see much of a difference. By the time you get to the muffler, the rest of the exhaust is the main restriction and you will get the most performance out of the biggest, freest flowing muffler you can find.
You actually want as little back pressure as possible. It's a frequently misunderstood thing and I don't have the time to type out a good explanation. The short answer is that you want the highest velocity you can get and the lowest back pressure. That being said, I'm not very familiar with tuning carbs and a decrease in back pressure can result in the engine running lean and having problems if it isn't tuned properly.
Another thing I should mention is that trying to add power to a 4AF is a long, expensive, unrewarding road.
If you don't like the air filter cover, try painting it. I did that to mine and I think it looks quite a bit better in the engine bay: