I don't think you're going to miss on either.
Will McDonald broke Iowa State's sack record twice while playing out of position, and Forbes owns the NCAA record for most career pick-sixes. Then they both lit up the Combine. They're total studs, and size is the only question you can hold against them.
Between the two, McDonald concerns me less, because we've seen Dan Quinn use 240, 250 lb speed rushers effectively a million times. Parsons is around that weight right now, and Randy Gregory was too. There's a key spot in Quinn's scheme for Will McDonald shaped athletes.
Forbes is a tougher sell, because Quinn likes big corners. He has the length Quinn covets, but it's weirdly matched to very literally the skinniest CB build ever. On Mock Draftable, Forbes has 70th percentile height, 92nd percentile wingspan, 78th percentile arm length, and 0th percentile weight. I don't even know how that's possible. Quinn usually likes his corners to be pushing 200 lbs, and Forbes weighed in at 166.
His ball hawking play style fits our defense, but is that weight going to be a dealbreaker for a coach who loves using thresholds to find CBs? It's at least a question I have. There's no doubts I have with McDonald.