I'm unsure about insurance etc and it most likely is based highly on geographic location, driving record and age.
As for the physical cars, I have'had both and driven many others. Regals and Bonnies share the same basic drivetrain. The Regal will turn tighter while still feeling roomy in the cabin. Many of the features one looks for are in the Buick. Meanwhile the Bonnie has some creature comforts not found in the Regal, like the SSEi seats and HUD.
Under the hood, many more aftermarket parts are made for the W body (Regal/GP/Monte) than the Bonne, both in exterior appearance and suspension. Not to say you can't do something similar with the Bonnie, it simply takes more research and time.
Do you have any specific questions mechanically etc?
BTW if you get the Regal, add a HUD from a GP.
00 Regal GS: All kinds of cool stuff.