My experience with Fax Over IP is that a good quality ATA makes all the difference and is much more important than whether you use T.38 or uLaw. If the ATA isn't capable of converting the analog fax signal to digital and preserve all the detail, the fax won't work no matter what else you do. I have found that the Cisco and Grandstream single-device ATAs are terrible at converting analog to digital in a way that allows fax to work. The Grandstream ATAs have a noticeable overdeviation and makes normal voice sound distorted, even when you turn the transmit gain all the way down (it just sounds quieter, but still distorted).
The Obi ATAs (Obi 110 or 200/etc) do the conversion very well. I've had zero problems faxing over IP when using them, even with uLaw. If you want to continue using your fax machine, I highly recommend them.
I haven't tested the multi-port grandstream ATAs, but they cost considerably more and so I'd expect them to have better analog to digital encoders, but that's just a guess. I believe Obi also makes multi-port converters and I'd expect them to be just as good as their single-port devices...