Video quality wise, it's a hard call. The HV30 has Canon's well established large CMOS sensor (1/2.7", the same one in my HV10 camcorder, which I put in the same class as my pro/prosumer class Sony HDV cam), it offers the fairly standard HDV MPEG-2 encoding, which is easy to deal with on most modern video editing toolchains. True, HDV is "only" 1440x1080, but the HV30 offers 60i, 30p, and 24p recording, which supports video, internet, and film-school use.
The HG-20 removes two of the problems I had with HDD/HD cameras: it can also record on SDHC cards (thus eliminating the "what do I do when the HDD is full" problem), and with a 24Mb/s full HD mode, you will find that at least in some cases, the video quality can meet or even exceed that of the HV30. On the other hand, they're using a slightly smaller (1/3.2") CMOS sensor.. it has good reviews, but I think the HV30 will have the edge in low-light. Don't get me wrong.. Canons are the best low-light consumer models around.. I just think the HV30 will outperform the HG-20/21.
Editing-wise, AVCHD is slow, even for applications that support it (I can edit it in Vegas 8, but it's painful, particularly in full HD, except on my Quad Core2 machine.. and even there, I would use an intermediate CODEC for any complicated edits.. MPEG-2 editing is fast these days, even on my relatively slow Athlon 64x2 machine). If you're married to Premiere Pro, make sure it supports the HG-20 before you go that way... AVCHD is new enough that some applications don't handle some implementations yet.
Features-wise, if I were interested in the HG-20, I would definitely opt for the HG-21.. it's got twice the HDD space, but more important still, it has a viewfinder... I don't relish the idea of the LCD panel as my only way to use the camera in bright light.
Both are excellent cameras. Neither is going to make you happy with the built-in mics.. you're going to need an add-on, if you use in-camera audio at all (I always do record with at least one camcorder and good external mics, but for any serious project, I usually have other recording devices involved). Even a small external mic (Canon makes a few that can get power from the camera) will be a big improvement over the built-in... and the most important single thing you can do for sound.
Another concern for serious videomaking is the ability to do anything manually... focus, exposure, audio levels, etc. Both models do offer a decent set of manual exposure and focus controls. Unfortunately, like most smaller camcorders, they're nearly all accessed via menus. The HV30 has an edge here, in that it offers a small nub for manual focus, similar to the nub used for zooming (they're nowhere near each other, so no confusion). With that said, I much prefer the lens ring on my Sony.. I can assign that to either focus or zoom, and it's much more natural than messing with the smaller controls. With that said, neither of my others have a lens ring, and do manage... it's a personal preference, not a deal-breaker.
The HV30 has an edge in some extra features, like offering zebra striping, peaking, and grid overlays in the viewfinder, which are not available on the HG20 (not sure about the HG21). I use these things when I have them.. you have to decide if they're important or not.
Both have the Canon-specific accessory shoe (great if you have a Canon accessory, annoying if you don't.. you'll need a adaptor to mount a conventional shoe-mount microphone on either of these), both offer a 1/8" stereo jack for mic input (you'll need an dynamic mic or a condenser with battery supply.. most consumer on-camera mics will work). They both give you on-screen audio levels as an option, and both support manual audio level setting.. again, via menus, but manual levels can give you much better results in certain situations.
I still don't advocate using the camera as your only recording means, unless you're doing a one-man run and gun documentary or something.. for anything serious, get a sound guy. But it never hurts to have the best audio support, anyway.