That's a tough one. The Tascam, I bet, is more robust, based on the way each company has built stuff in the past. I use the H4n, and find it a very usable unit. I've handled the Tascam, but never used it. The Tascam only records 2 tracks at a time, unlike the H4n, and only records in 16bit/44.1 kHz. It gives you 8 tracks out, to the Zoom's 4. The Zoom can act as a computer controller using the included Cubase, or most other software, which should give you at least 16 total tracks out. The Zoom can record 4 tracks simultaneously, but two of them have to be the onboard mics, or an external stereo mic.
The H4n can do two track recording in higher bit depth/resolution, up to 16bit/96kHz. The X factor with the Tascam is actual EQ knobs and analog panning. It's also larger, physically, than the Zoom, which makes it easier to use and harder to transport (The Zoom fits in most guitar cases).
I think it comes down to preferences and applications. If you like your hands on a lot of dials, and you intend to use the unit mostly for band practices or small gigs, I might choose the Tascam, especially if you're not using it with a computer. If you intend to do processing in a separate computer, and you want 4 tracks for drums, I like the Zoom. Its physical dimensions and more extensive onboard FX (amp sims, compression, and more) make it a superior notepad for a traveling recording artist/guitarist. For my purposes, I prefer the Zoom, primarily for its versatility.
What I really should have said is- other- Zoom R16. Yeah, it's $400, but blows the others away. Instead of the PDA of a recording artist, it's the netbook of a recording artist. It does everything the H4n does, but adds 8 track simultaneous recording, 16 tracks out as a standalone, and real faders.-Richie