I've been trying to make the same decision, and here's what I've come to:
Driver updates? nVidia updates their drives like ever 14 minutes...just use the reference drivers.
I'd get the Hercules/Guillemot because they clock their RAM at 183Mhz over the standard 166Mhz for an MX, so they have a performance edge at about 800x600x32 and higher. Asus gives like +5 core and +5 mem...but at 8x6x32 and up (with SDR SDRAM at least) you are memory limited so the Hercules is a better boost.
Then just use the nVidia reference drivers and your fine.
As for TV-out quality, I'm pretty sure that both cards use the Brooktree chipset, and while it sucks horridly (I've been spoiled by the Rage Theatre chip, and a hollywood plus) it is the standard for TV-out chips. ATi's newer cards don't use Brooktree, and I don't think Matrox uses it either...but any other TV-out card you've seen would probably use it. So either TV-out would look pretty much the same and the same as just about any other card you've seen.
Asus includes both S-Video and Composite output on their card, where as hercules includes only S-Video and gives you an S->Comp adapter. I think the Asus way is probably a bit better but I don't think it matters much, any distortion would be covered by other factors.