If you leave Sprint, that means that you will be porting your number to Ting, or cancelling the account all together. I believe there is a grace period in case you come back to Sprint, though, where they won't charge a re-activation fee.
Basically, if you need the line to be re-connected, or if you open a new line, then you will be charged the activation fee. You may be able to call in after the fact and try to sweet talk your way into getting reimbursed, but that's not guaranteed.
If you just want to test Ting, the "safest" route would be to suspend the Sprint line for a month, get a cheap phone for Ting, and see how it is. There really won't be too much of a difference on Ting in terms of coverage, except that you won't be able to roam.