None of Vauxhalls FWD gearboxes come complete with LSD from factory. A quaiffe item can be fitted to either box which will obviously improve traction no end.
As for the wheelspin question, you get the same amount of slip between an F20 and an F28. The only reason you should opt for the F28 though is for its strength. If your not running big power stick to the F20.
In adition the F28 will have to be converted to run 2wd if you choose this box though so the cost soon starts to mount up rather quickly.
If you need a box converting or diff fitting just give me a shout.