Hi, that's a very complicated question. If you type in strings in the search engine, you will get more info than you can read about different strings.
to answer your question about Aquilas, they generally make an inexpensive, laminated uke sound way better than the cheapo strings that sometimes come on them. Other than that, it's a uke by uke outcome. Many vintage Martins seem to love them, most solid wood instruments are case by case. With many ukes, Aquila Nylguts make them sound "brashly" bright and somewhat annoying. With others, they provide a measure of brightness that a particular instrument can handle by virtue of their construction and the nature of the wood used. If you want a greater measure of "mellowness", they may not be the best choice as other nylon strings can provide a great range of tonal possibilities. That's why so many of us are always trying new strings. You'll probably get several responses, but would increase the likelihood of that happening if you stated what uke(s) you have and how you would like them to sound. What strings are on it now, and what you don't like about them, etc. good luck