1) What are the questions that a client should be asked for choosing which version to use?
It's subjective so can not answer for you. Usually, you can start asking where they have their existing database, would they be okay hosting their marketing database in cloud etc.
2) Do all the users of ACC V7 could migrate to ACS? if not, what are the conditions?
It will be a manual process, there is nothing that automates this. ACS and ACC are two different ecosystems, Adobe is working on a connector to connect these two(it's working for some case but comes with limited features)
3) What are the other advantages of ACS in addition to being user-friendly?
It comes with integrated Dreamweaver so it's easy to build email template. You have built in dynamic reporting which is really nice.
1. Are any of the AC client companies operating Standard and Classic side by side? For example so that workflow's are created using ACC v7 whereas deliveries are executed to those target groups using ACS?
2. Can you specify what you mean by limited features here: "Adobe is working on a connector to connect these two(it's working for some case but comes with limited features)"?