Planes are like most things... whether or not they're good depends on what you want to do with them. If you're a beginner, the Mustang will be a good plane to save for later.

I'm not familliar with Seagull, but the P-51 design (if followed well in the model) is a good one in terms of producing a plane with some aerobatic capabilities, a good appearance, and reasonable controllability for an experienced pilot.

Have you graduated from first trainers and gained some proficiency on a low-wing sport plane yet?