Never having stayed at either but been to both more than once, here's my .02:
For shorediving Buddy's is easier. They have a drive thru tank pickup at the back(street) side of the property. At the Plaza you drive to the SW corner of the parking lot near the Toucan fill station and carry tanks about 40-50'.
We boat dove with Toucan. They were great to dive with, mostly left you alone. Buddy's definitely has bigger boats/more divers per boat. If you plan to boat dive at Klein, Buddy's is definitely closer.
Buddy's diveshop is off their dive dock. Plaza's is near their dock on the south end of the resort. But there's no diving there. Plaza's dive site is 18Palms. It's near the north end of the resort. They do move tanks to the area for shorediving. From one of the Lagoon condos by the diveshop, it'd be a healthy walk with your gear.
We went to both diveshops, Toucan's was better. IMO it's the best on the island.
If you stay at Buddy's ask for the Lion's Dive area, it was a separate resort renovated just b4 Buddy's bought it last year. Since it's adjacent you have to walk across both properties to the Buddy's diveshop/dock now. But resorts run perpendicular to the water in that area so it's not far.
IMO Buddy's is more for divers, Plaza is also a resort/convention center for tourists. I've read better things about the Buddy's staff friendliness/helpfulness here.
Plaza has a beach with a bar, As I recall, they also had some watertoys there, and volleball. Sometimes there's a Friday night bbq with live music. Nice big pool. Buddy's has no beach being on the ironshore and I've never seen their pool. Someone mentioned here that the crowd at Buddy's Bar can be loud later at night.
Both are about the same distance from town more or less. Neither is walking range. Plaza is 5 minutes from the airport. Distance to divesites is also a wash, each is 5-10 min. from the good shore sites in it's area. We stay at BelMar condos south of the Plaza, they're nice, almost new and quiet. I posted Google Earth images showing both resorts
here.
Bonairetalk has resort info also.