I use WriteUpp, but originally explored both. My Therapy tracker has the advantage of the client sharing as mentioned, which I would really have liked, but it was the price that swung it for me too. You can get parental consent and it has an online diary, booking system and you can use it for invoicing which links with some accounting software, but not Quickbooks which I use.
I also like the way the notes are presented so you can scroll back through the entries in a continuous page. They don’t have the complete range of phonetic symbols, but I don’t know what MTT has for this.
Pretty happy with it although none of these systems are perfect!