I can speak from personal experience with the M-Audio MobilePRE and the E-mu 0404|USB.
Personally, I don't tend to recommend any interface unless you're getting at least two input channels, and then really if you're getting 2 mic inputs and at least 1 Hi-Z input. When I originally was looking at small interfaces, I passed over the original Fast Track in favor of the MobilePre for that reason.
That being said, there are things I like about the MobilePre and things I don't like. For starters, at some point I found myself wishing I had MIDI I/O. For the sake of buying a little beyond your current needs, that's something I didn't realize would would happen at the time. The drivers are also somewhat primitive, but I also found them to be pretty stable. However, unless this changed in the latest generation of the device (I have the original MobilePre), it's limited to USB 1.1, so you're a little contrained in terms of high-end audio (wasn't a big deal to me). I also really like the fact that it is powered through the USB port on the computer.
As for the E-mu 0404, I ended up purchasing this for 2 reasons. The first being the desire to have a small device with MIDI capabilities. The second one being that it was a fairly inexpensive way to acquire both a "bigger" interface than the MobilePre (more options for inputs, including MIDI), and also that it comes with what was, at the time anyway, the best OEM software package I could find. I also was curious about Sonar at the time (I had Cubase already from another, larger interface) and the 0404 let me knock out both issues with one stone. The preamps have slightly more gain than the MobilePre, but sonically they're pretty similar. The E-mu is also physically much larger and requires an external power adapter, so sometimes I wish it was bus-powered. On the plus side, it's a USB 2.0 device, so it can handle larger data rates for better quality digital audio. The drivers are nothing to write home about in terms of improvement, but they work and are pretty stable based on my experience. The soft limiter may or may not work for what you want to do with it (I happen to think it sucks, but I can see it having applications for different types of music). E-mu also took advantage of the larger physical size with a better overall layout as well, in my opinion.