Remember of course you will get another 10-15 years of life from the OEM Cat!
I too almost found myself in a similar position of buying a new Cat, GBP1500 here but thankfully I reviewed UK law on emissions - it was well worth it as I discovered that my '93 can pass non-cat limits so that's GBP1500 to spend on car toys
Must be some quirk of the Eurodirective
However, I had the same thoughts as you before my research, new OEM Cat or patent?
Over here a patent part is around GBP380 but my concern was for it's life? For the cost difference I think I would have voted for the cheaper and took the chance. But if it only lasts just in excess of the guarantee (one year) that's expensive!
Never did make up my mind.
Lea