Hold or Sell in the Bronx

[Michael M.]

Hold and rent out or sell?

I was offered 300K more than my house is worth (a 2 fam).  Currently, my mortgage is 2000 a month and my rental income is 4600, which leaves me with 2600 a month income after mortgage.  My calculations tell me to sell and reinvest the gains into the market and buy a 3 fam for the same price that my home sells for.  The rent vs. sell calculator tells me to keep the house and rent out (http://renterswarehouse.com/rent-vs-sell-calculator/).  Am I missing something, or is it because the calculator assumes I am getting market price?