First, I would be happy with either (or even Bradley Beal)...all 3 would be a significant upgrade IMO over what we currently have.
What I like about Lamb....His athleticism...his size....his wingspan...his ability to score
What I like about Rivers...his moxy....his ability to breakdown his defender and get into the paint...his willingness to be the go-to guy (and deliver)...
So, I have been advocating drafting Rivers for awhile b\c I think his game along with his desire to be that go-to guy are going to translate to the NBA....but one thing brings me to pause a moment....I think he is going to be an isolation player in the NBA....much like he was at Duke, and much like Roy was in POR...probably not as good as Roy, but who knows...but the question I have to everyone is, do you want to go back to that type of offense? I do not...I don't think it is ultimately conducive to winning an NBA title which is what this is all about...I don't think having a team stand around and watching one player go ISO all the time is a path to the NBA championship.
Of course a lot of this will be dependent on the coach POR eventually hires, and if they draft Rivers, his willingess and ability to play within a system...and he woun't have to do it all the time, and certainly you do want players who when you do need a bucket, you can give the ball too and llet then create for themselves or other players.....
I think both will be good NBA players....I am intrigued by the possible length POR will have with a Lamb, Batum, Aldridge trio...add Henson\P.Jones to that mix and a FA PG, and you might have something there...a tough team to match up with....Rivers though IMO likely will end up the better scorer\ISO player of the two....
So bottom line...I would be happy with either...but in the thread of rating prospects I recently listed them as Beal...Lamb...Rivers, all very closely grouped, but I would take Lamb...the guy is just a freak athletically and has good measureables, those generally translate pretty well....