I would suggest that first you check to see if Fannie Mae or Freddie Mac Own/Insure your loan. You can check that at the following two websites:
Fannie Mae: http://www.fanniemae.com/loanlookup/
Freddie Mac: Google "Freddie Mac Loan Lookup" and it will be the first link (they won't let me post 2 urls)
If it is owned/insured by one of the 2, then you can potentially look at doing a HARP loan.
If not, which would be my guess since you didn't have mortgage insurance, then I would lean towards the FHA loan. Based on what you provided, and assuming a loan amount of $150,000 the payments break down as follow:
FHA 20 Year Fixed @ 3.99%
Principal & Interest: $917.26
Mortgage Insurance: $143.75
TOTAL PAYMENT: $1061.01
(estimated APR for disclosure purposes: 5.170)
20 Year Fixed @ 6.00%
Principal & Interest: $1074.65
(no APR estimation because this is not an interest rate I am quoting, just running payment)
So the FHA loan gives you a lower payment even with the mortgage insurance. And if you prepay your mortgage, then you should be able to get that mortgage insurance to drop off after the 60 months that FHA requires you to pay it. Also you should refer to a CPA regarding mortgage insurance being tax deductible.
HOWEVER, FHA has announced increases to their Mortgage Insurance Premiums (both Up Front and monthly) that start for FHA Case Numbers ordered on or after April 9th. This would increase your monthly payment to an estimated: $1080.32.
Lastly, keep in mind that FHA will only allow up to 97.75% LTV, so the appraisal would be the wild card.