Is the 7/1 ARM justifiable in my case?

Currently I'm in a 30 year fixed at 5%. Will probably stay in this house for another 5-7 years before moving on to a bigger one. I can get a 7/1 ARM at about 3.375% which will save me about $100/month, taking into account all variables including deductible interest and PMI. My mortgage balance when I sell 5-7 years from now would be about the same with the 7/1 ARM as my current mortgage, assuming about $2,500 of refinancing fees is rolled into the loan. Would you guys refinance or are the savings too low to justify the interest rate risk on the back end in the case where I stay in the house longer than planned?