I went with PdaNet, since I had a copy from my old TREO and on my Storm2 and DM v5 it all played nice. I did do a trial on Tether recently but couldnt find a reason to change.
I dont remember, however 30 days seems to stick in my mind. If you do not have PDANet or TetherBerry try them both and let us know what you think and which you deciede on, please.