Watkins was an all-or-nothing gamble to provide a weapon for EJ, who, at the time, hadn't really shown any progress. Whaley used that year's first and traded the next year's first to move up.
Diggs is a proven vet who's still young and has a reasonable contract, and Josh has shown himself to be a competent NFL QB, continuing to make progress through his second year, so it's not a gamble at all. Beane traded a first round pick, which he might have used on a WR who's never played an NFL down, and some lower picks that may or may not have panned out.
In short, the Watkins deal was high priced, high risk, high potential reward.
The Diggs deal is lower priced, lower risk, and at least as high of a potential reward.