I have reservations due to a long history with Samsung (awesome hardware, lame software), but I'm pretty solidly in the Epic4g camp right now for the following reasons:
* Keyboard. The Hero taught me to loathe and despise soft keyboards. Admittedly, Palm/Access threw a curveball into my equation with the release of real, honest to god Graffiti-1 last week, but for now I'm standing firm and demanding a keyboard, if only because it doesn't eat the lower third of the display.
* GPU acceleration. Yes, Qualcomm has open-sourced the acceleration api (sort of), but HTC has shown that they won't regard it as a priority (or even a 'to do' item) until Sprint does... and Sprint doesn't.
* Samsung's kernel has the proprietary binary drivers implemented as proper .ko loadable kernel modules, instead of the monolithic, GPL2-violating binary blob HTC $hits on the curb and walks away from.
* thumb/indexfinger-friendly power button on the side, instead of on the top. For single-handed use and back-pocket carriage, I think that's ergonomically superior to the Evo's top-mounted button. It's a small detail, but one that becomes *really* important when you're talking about a button you're going to be using *constantly*.
Touchwiz might suck compared to SenseUI, but both can be irrelevant the moment the phone's rooted and reflashed, anyway. Anything SenseUI can do, $10 worth of thirdparty widgets from Android Market can do equally well or better.