I haven't really paid much attention (slow is slow), but afaik phonenet is going to be faster with a theoretical maximum around 230kbps. Depending on what you're connecting to, you'll probably end up with a 115200 baud connection with a null serial link. Assuming you could get an equivalent speed null serial connection, the overhead of ppp and any other encapsulation will probably have a great deal to do with what exactly you're doing. My suspicion is for IP traffic, the overhead of macip through a gateway is going to be both lower bandwidth and higher latency than ppp to a modern machine (for an equivalent speed link). AppleShare is a different story.

I think I only synced up at 115200 baud. I don't worry too much about speed when using 15yr old gear.