all the traditional marker bindings SUCK HARD for any kinda park riding. as you said, they are heavy as hell, have zero elasticity, a large footprint, and are way too complicated to be reliable.
get red of those ASAP.
as for replacing them, the PXs are OK, the jibs and all the other cheaper PXs/axial 2s are realy heavy, and the only real benifit of them over your markers is the release. they have a better hold and more elasticity, but they still have a LARGE footprint, tab/track heel, and wont be that durable. the higer end ones with the aluminum construction (where the spring body hinges off the plate) are definitely better, and worth the extra $
salomon STHs and marker jesters are 2 other good clamps, but neither has the elasticity of the rossis.
what i would really reccomend, however, is jumping on ebay, and finding a pair of Look Pivot binders or first gen Axials. they are dirt cheap nowadays, are bombproof, have the elastisicty in the release (nothing has a smoother more consistant release than pivots), and a tight mounting pattern in the heel, which will give you a hinge point closer to your boot heel for better pop. the other option is some FKS 120s. same toe as the pivots, but with the race heel. the pivots are much easier to own, and require alot less maintnence, and in my expierence are more durable. the FKS heel has more forward pressure and a more direct connection than the pivots (they ski a lil better), but the release is not nearly as smooth.
http://cgi.ebay.com/Look-Pivot-8-0-turntable-binding_W0QQcmdZViewItemQQ_trkparmsZ72Q3a570Q7c66Q3a2Q7c65Q3a12Q7c39Q3a1Q7c240Q3a1318QQ_trksidZp3286Q2ec0Q2em14QQhashZitem230310037753QQitemZ230310037753QQptZLHQ5fDefaultDomainQ5f0QQsalenotsupported
http://cgi.ebay.com/THE-BEST-SKI-BINDINGS-EVER-MADE-LOOK-PIVOT-9-0_W0QQcmdZViewItemQQ_trkparmsZ72Q3a570Q7c66Q3a2Q7c65Q3a12Q7c39Q3a1Q7c240Q3a1318QQ_trksidZp3286Q2ec0Q2em14QQhashZitem330289920902QQitemZ330289920902QQptZLHQ5fDefaultDomainQ5f0QQsalenotsupported
(pop those off the plate, theyd be money
feel free to PM with any other questions!