Advantages of Sympli and Zeplin over Avocode (for layout purposes and other frontend)?

Question

Layout

I tried Avocode and Zeplin, but while I was trying and comparing, I managed to hear and read a lot of praises about Zeplin, Sympli, Invision, Marketch (for Sketch).
It is very difficult to always choose, the topic in no way calls for holiness, the question is specific: which tool is more suitable for the frontend, or what is the killer feature you noticed that another tool does not have?
Thanks in advance.
PS Or is Avocode really the best?