the 2 & 3 are how many strands are twisted together.
visit the website of 'Superior Threads' they have free charts you can print off telling you what needles to use with what threads, what different threads are good for, differences in types, weights, fibers of threads- they are a valuable resource. I use a lot of threads from Connecting threads- I like most of them just fine- I like YLI silk threads- I haven't tried the one you've been using- I have nothing local selling YLI threads. if i'm not piecing with a connecting threads thread I am generally using aurifil or silk. the 2 you are 'comparing' are both 50 wt threads- just one has 2 strands twisted together, the other has 3 strands