'06 JCW with a eBay race header. I'm throwing on a Vibrant resonator as its too raspy for my taste. I also would like to put in a cat as there is none. I've read a handful of threads and the OEM seems the best option but I don't want to chop it off if I want to go back stock.

Do I want a 200 cell count
http://www.ebay.com/itm/2-5-Magnaflow-Universal-59956-Catalytic-Converter-High-Flow-Spun-Metallic-Cat/170862884170?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20160617090737%26meid%3D4aa93db2b21f4a49984437f6a1faf395%26pid%3D100005%26rk%3D2%26rkt%3D6%26mehot%3Dag%26sd%3D171078843109
or a 400 cell
http://www.ebay.com/itm/171078843109?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

RMW will tell you most aftermarket units will either melt or crack due to heat and harmonics...I know if a few 250 hp gen1 cars with the stock units selected onto the shorty headers....if the OEM is good, you loose almost nothing by using it....the gain is so slight, and long life. Aftermarket work for some people....but many get a light that will never go out, leaving emissions tests and inspections all but impossible.
Not sure which universal unit...but like I said...OEM is on 250 hp cars if it's good....
Unfortunately the specific individual from Philly I'm thinking if is in the other board...left here...but it's his daily driver.

Ditto, your loss is minimal, if at all, and OEM is the only real option for trouble free longevity.

Ok back to stock. Thanks

And as a general note...
The "spun metal" style converters are an older style....usually ok for older cars... generally pre 2000 cars are ok for them....but if a pretty crappy desgin...but cheap...but not effective enough for increasingly clean and pickey emissions systems.
Newer cars usually use a metal or ceramic unit...the MINI OEM unit is metal.

I got this ab catayletic 201 from amazon and have been very happy with it. I have just about 10,000 miles on it now.

Nik

Well in that case the 59956 is a spun metallic cat and the 99656HM is a heavy metal cat. So I guess in my case I should go with 99656HM if I were to go that route.

If you need to change it out, yes...but like I said, the OEM is your best option if it good imo.
Typically my shop guy has told me aftermarket cats tend to last 2-3 years, max....unless you buy a very well built CARB approved unit with a long warrenty...
If you do go aftermarket, my suggestion is to note the psi boost (if you have a boost gauge) you get right after Installing it...then later if you have any issues...check it...if it has suddenly started to go up...that is a sign of a failing cat...the flow gets slower as the honeycomb melts or cracks.
Sometimes folks sell OEM headers that are cracked or when they go aftermarket with the OEM cat still attached...another option.