If you're dead set on staying with burton you could check out the Hail. Good boot for the cash. It's hinged cuff will let you flex so you can really tweak out your tricks in the park.
But if you're willing to use something besides Burton then check out the 32 Lashed. One of if not my absolute favorite park boot. I used them with my Union Contacts and my Lib-Tech Skate Banana. They fit great, really comfy, and not too stiff to let me flex in my bindings.
If Budget is what you're looking for may I suggest you check out some online shops like Evo, Dog-Funk, and Eternal and look at Forum boots. With Forum heading out of business most of these stores are blowing out their products at ridiculously low prices. I think a buddy of mine got some nice pair of soft Forum boots for park riding for like 90 bucks at Dog-Funk a week ago with free two day shipping too. Check that out if you're looking to save cash.
Good luck, but again I prefer 32 boots for park especially the Lashed.