This is more of a personal questions. What would make you feel more comfortable. With some the fixed rate gives them a piece of mind. Granted it will readjust quarterly after that. Then you could examine the options you have at that time. It sounds like if you increase your income and value in the property after you have the property awhile you will qualify for more competitive loans tied to libor.
Remember the prime is guestimated to go up. It has been going up inorder to improve our markets. Take the stock market into consideration. After the shock we experienced with the 9/11 the govt used its last resort to boost the economy by compensating with the primse. The lowest it's been in god knows how long... (Sorry I don't have my stats at home)
It's a personal decision to make... Look at the trends and I'm sure you'll be able to decide what's best for you.
My grandfather always said, " I"ve never been much of a gambler. I could never understand why someone would risk something for more, with the likely hood of chance/luck being against them. Then they end up with less then they could have originally had guarantee'd." (he was a simple man)