If you're asking which I like better, and would rather have - it would a Block II.
It's a little bit heavier, and doesn't balance as well, but it's really not that bad...
However, the Block II basically does everything better "all around" than the Block I - FF barrel, more space for grip, and add on items like a CNVD without having to re-arrange the whole rifle, bipods if desired, etc. without feeling crowded.
If you don't have/don't have access to those things, it's perhaps a non-issue, but I prefer to be able to have the capability to do so if circumstances change.
The longer gripping surface of the RIS II also make it easier to manage the weight of a suppressor at the end of the long barrel.
That being said - as I do not get a RIS II equipped rifle for work, in my particular situation, if you told me, "you can only keep one," it would be a standard, USGI M4 MWS clone, much closer to the Block I... though I reserve the right to accessorize it as I see fit.
~Augee