That is a lot to consider: Aside from all the investment, tax, emotional, and expense considerations there may not be a choice depending on your financial situation. FHA, FNMA, & FHLMC all have very specific guidelines and requirements in order to finance an additional property. Here is exerts from HUD and the mortgagee letter that specifically address's this issue
MORTGAGEE LETTER 2008-25
It literally states
"Due to FHAâ€™s concern that some homebuyers in these transactions may attempt to provide misleading information regarding the rental income of the property being vacated to qualify for the new mortgage, FHA is instituting underwriting guidance designed to assure that the homebuyer can make payments on the full debt service of both mortgages. "
Its a lot to consider I hope this helps.