This is the critical question. Technically, downloading modified APKs exists in a legal gray area. You are not "hacking" Google's servers, nor are you downloading copyrighted movies. You are simply altering the client software. Google has the right to block these apps, but they rarely target individual users. Using this does not get your Google account banned, but it violates YouTube's Terms of Service (specifically section 5.1 regarding modifying the service).
In the absence of an official changelog, the "top" build is usually the newer one (740) for security, unless the user has a legacy device, in which case 617 remains the "top" choice for compatibility. The deep truth: smart youtube tv 617 740 top
Here is why this string of numbers is the "Holy Grail" for users with older or ad-bloated TVs. This is the critical question