If you want a new modem and you have an excuse to buy a fast one, I recommend either of these units. If you only connect at 2,400 bps, you can probably go for a cheaper 2,400 bps data/9,600 bps fax modem. Otherwise, the price is right for what these modems provide in terms of the fastest data transmission commonly available and the added send and receive fax capabilities that businesses find useful.
That doesn't answer your question, though, about which of the two to purchase. In most ways, the modems are similar, so it comes down to specifics. For my uses, the PPI gains a slight edge because of the one quirk with the Supra in talking to the WorldBlazer and the neat LCD display. I seldom use faxes if I can help it because I don't approve of them, so PPI's Quick Link II Fax software doesn't bother me as much as it would otherwise. Finally, I'm unlikely to travel with this modem at all, so the PPI's larger size doesn't bother me. In summary then, I think the PPI is a slightly better primarily-data modem for desk use. The Supra has far better fax software, is cheaper, and is much smaller (although it runs hotter because it uses the metal case as a heat sink). I took it to Boston for Macworld and found it a good traveling modem, though certainly not as small as some. You won't go wrong with either modem, but it might be worth checking into other modems that these two companies have released in the meantime. Supra has an internal PowerBook modem, and PPI introduced a tiny pocket modem, both of which might be better suited to travelling if you do a lot of it.