I use Notary Assist. So far, it has worked for me. It has everything that I have found that I need to keep up with my billing.
If I had one complaint I wish that it could automatically send invoices. What I mean is I would like to be able to set up a time frame for those that are late in payments. It would also be nice to batch them together. This is just my thoughts and basically I believe personal preference for these options.
But I will stand by advise that Notary Assist is a great option.
However, to be fair I have not tried Notary Gadget. Maybe someone else can respond with there advise.
Hope this helps.