Refinancing..should I choose fixed or 7/1 adjustable?

I'm hoping to save enough money to build a new home in about 6 years, but you never know how things could go...our current house could possibly not sell..we may need more time to save enough money etc. We are refinancing and I could get a 3.875 7/1 or a 4.5 fixed. Wondering if I should go with 4.5 in case we end up at our current home longer than 7 years. I don't think it would be a good idea for us to refi again in 7 years as we don't want to stay with our current home very long and also rates could be very high at that time and we don't want our rate to adjust too high. At the same time, if we are able to sell our current home and build our new within 7 years it would be so much better for us to choose the 3.875..mre savings and more paid toward current principal. Not sure which to choose. Opinions please???