I have a 3-wheeled one and don't have any trouble with it tipping over. 3 wheels supposedly make it easier to go over rougher terrain such as grass. I like mine a lot and so do my baby's. Put 3 of my Yorkies in there at a time. My Pommie wants to ride all the time although he needs to walk more than his tiny brothers and sisters.
This is the one I have:
PetZip Luxury Monogram Trim Pet Jogger or Stroller A701
~Joanne~
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Joanne

Eli

Chipper

Eva

Snowboy

Rosie(R.I.P. 1996-July 29, 2013)

Cocoa(R.I.P 1998-April 26, 2012):