This is where reasonable people can disagree, as they say. Herrera is still just 26 years old with a career .763 OPS, two near-Gold Glove seasons and 10.7 WAR in four seasons, only one of which, the last one was bad. He's been an all-star and looked like one again for two months just this past year.
Franco is the same age with a career .738 OPS, consistently underwhelming defense, and 2.5 WAR in four+ seasons. He's looked like an all-star for a few months here and there too but his lows have been far, far lower. I just don't buy the idea that Odubel is cooked and Franco is on the rise, but either way it's no contest who the better player is. And they both seem to be flawed in terms of intangibles, if you rate that sort of thing.
Signing Manny improves your defense at two positions (3B and RF). Signing Harper means bad defense at 3B again, and his own defensive play is just as questionable as Herrera's for the moment.
The other x-factor is the trade market. Herrera has a lot more value than Franco, in part because of the cost control. If he is a better starting point for a package to get a pitcher, great.
Also possible they sign Manny and still sign another OF which might be the best scenario.