Eric,
The models with the Hi-Rail wheels are like your current PS-2 models; made with deep tin plate flanges. The models with Scale-Rail wheels have smaller flanges that look more realistic but do not track well on Tin Plate track. Scale-Rail wheels work better on track like Atlas O with flat top rails.
And as Clem mentioned, the Scale-Rail wheeled engines need larger radius curves so check MTH's web site for requirements.
If you have Tin Pate type track (rounded tops), I would also recommend you buy the engines with Hi-Rail wheels.