Hi, it doesn't matter which one you choose, its just down to personal preference. The Mongoose system seems to be very popular. What I would say though is go for a full system from the turbo back, with a 200 cell sports cat. The bigger downpipe makes a noticeable difference as you will notice the turbo will spool up quicker. It also enhances the sound of the exhaust and turbo. Likewise, the sports cat will also enhance the sound as it is more free flowing. Don't be tempted by the decat option, although much cheaper than a sports cat, you will fail the emissions and your insurance will be void! You could go for a 100 cell cat as that will be more free flowing than a 200, but you will be sailing much closer to the wind with emissions, and after a year or two it may even fail. Therefore the 200 cell is a much safer option.
If cost is an issue then check out ebay and the classified sections of this site and forums such as www.focusstoc.com and also www.ST-OC.com as there is always someone selling exhausts. I did this and saved an absolute fortune on my downpipe and sports cat. I did buy my cat back system new though, but that was before I found all these sites!
Once you get the system on, it really brings the five pot sound to life with plenty of pops and bangs on the over run and in between gear changes. The systems you have mentioned will not be too loud either. I absolutely love mine - nice and quiet when pootling around town, but a nice bit of noise when you hit the loud pedal. Lift off the throttle and you'll get loads of lovely pops and bangs.
Hope this all helps, and let us know what you go for. Also there are plenty of videos on YouTube to give you an idea of what different systems sound like.

Nick.