I hope to write a comparison of my WhipMOD and HM-801 one day, but basically these are 2 very different players for 2 very different uses.
The 240GB iMod/WhipMOD with Rockbox is an insanely versatile and high-capacity player--it'll play just about every music format, with great features like gapless playback and the ability to delete files directly on the player. If you have a massive collection of files in various formats, and you value having the ability to have a lot of it at your fingertips without having to swap SD cards or transfer new music frequently, then there really isn't another option.
For pure detail retrieval, soundstage, and realism, I think the HM-801 wins. But the HM-801 has its own problems, like short battery life, large size, and limited storage (unless you're willing to buy and tote around several SD cards). If you don't have that large of a collection, or if you're willing to copy new music regularly whenever you get tired of the songs on there, then the HM-801 might be the better option. The HM-801 superior sound quality over the WhipMOD isn't dramatic IMO, but it is noticeable, especially with well-recorded music.
A 240GB iMod can really be had on eBay for around $300? I assume these are 4th gen., since the 5th gen needs an LOD with caps?