I have an IHP-140 myself and am well pleased with it. Which one is better for you pretty much comes down to what features you value most. I won't deny it, the Ipod is slimmer and has a somewhat slicker interface. Plus, all the cool kids have one these days. On the other hand, the Iriver has superior battery life, a far better wired remote, and considerably more impressive features(when I was buying it also happened to be around 50 dollars cheaper, and I like black cases better than white ones, your mileage may vary). I've actually had some use for the internal microphone, optical in and out jacks, and analog in and out jacks, so they matter to me.
Besides physical features and appearance, there are two other main points of contention. One is file formats. Both players can handle .wav and .mp3 trivially enough. The Ipod has .aac while the Iriver can handle .wma and .ogg. I have most of my collection in either .mp3 or .ogg(better and easier compatibility with *nix/*bsd and better sound quality) which made the Iriver seem like a better buy. You should keep your preference in mind.
Finally, method of interfacing with the host computer. The Iriver appears, to the system, as a straightforward USB2 hard drive with standard fat32 file system. Non music and music files are handled exactly the same. You simply use your platform's preferred file manipulation tools to drop files into the player and you are all set. I just use a hierarchical folder system to keep mine organised. Artists or genres are top level folders, artists or albums second layer, disks third layer, than tracks. Any system will do; but you might well want one if you are getting even close to 40 gigs. You can even, if feeling somewhat perverse, use Itunes(The Iriver is a standard hard drive, Itunes works on music collections on hard drives, and so forth). The Ipod pretty much depends on Itunes, of course, though I've heard tales of various OSS clones for linux. Some people like the convenience, some people tell horror stories about losing the music on their computer and not being able to get it back from their Ipod, your mileage may vary.
All in all, though, it is something of a matter of taste. Apple has, undoubtedly even by the admission of its critics, created a great MP3 player for people generally. Cute, slick, trendy, easy to use. However, it lacks a lot of things that a slightly geekier user base really would rather have e.g. longer battery life, better I/O, more standard interface and format support, superior recording options, no DRM nastyness, etc. Your choice.