Personally, if Im the Giants, I just got for Milroe in the 2nd. Daboll has had success with a "raw" athlete in Allen. Russ gives Milroe a minute to sit and learn. And by not taking a QB at 3 or by giving up so much capital to move back into the 1st, they limit the impact a bust QB has on them. But there upside is a QB for 3 years on a 2nd round contract(assuming he starts in year 2). If you're not sold and a better QB is available next year, you didn't invest so much in Milroe that you have to compound the mistake by passing on another talent.
I like Dart more, but it all depends on what the cost is to move back up.