I'd go with the cheaper option. If you can use your old cat do it. If you can get it out and give it a good visual inspection that would be quite helpful.
If it is plugged up try soaking it in a citric acid and water over night.
since the manifold is cracked as well, i'm gonna buy a new one. a used one would likely be cracked or crack soon and ebay isn't much better in price. the atp and dorman prices (~$200) are nearly the same, both w/ lifetime warranties.
the expensive bosman, catco, and eastern brands are REALLY pricey and w/out lifetime warranty.
bought and installed the 1 piece...much better acceleration, but still lacking in FE. i miss my 2000 prizm, which was yielding several more mpgs over the civic. no doubt the deep tread michelins are hurting me. gonna try to upgrade to 195* thermostat next. bought one at rock auto for >$6 shipped if anyone is interested!