Low versus high wing
Not sure from the original post whether we are talking about a training business or not, but if we are it seems there are two things to consider:
1. Is the type supported? Where I fly (CZ) Cessna singles are common and supported at pretty much every airfield. Pipers are very few and far between. Can you get it fixed if it breaks? How much will the spares cost and how long to get them there? Can the mechanic fit them? This ties to the comments on the (brilliant) Robins - every hour on the ground loses you money.
2. Choosing one high and one low wing aircraft will give your students the experience of both - invaluable in my limited experience, and a great differences training opportunity.
There is another decision to be made here - do you go for fixed pitch/welded gear versions of both, or the simple version of either, with the complex version of the other. If teaching at CPL level the second option will increase your versatility.
Good luck - it's a tough business to go into at the moment.