Based on final quality and editability.
Currently, HDV has been the reigning champ for quality for high-definition camcorders. And virtually all HD camcorders trump SD models on quality, even if you're shooting for delivery on SD (DVD, for example). The one place SD cameras, particularly pro(ish) models like the Sony VX2100 shine is low-light... you will still get better performance out of a top 3-chip SD model than a similar HD model in very low light. Any camera with a 24fps mode will also help in low-light.
Format wise, there's basically two in common use, at least until you're up in the >$5,000 range. HDV is, of course, high-definition (1440x1080) on mini-DV tape using the MPEG-2 CODEC and MPEG Layer-2 Audio format. AVCHD supports a wide variety of options, though it's based on H.264 (also called MPEG-4 Part 10 or AVC), which is pretty well established as the successor to MPEG-2 in may fields (video discs, digital television, etc). HDV is fixed at 25Mb/s regardless of whether you're shooting 1080/60i, 1080/30p, or 1080/24p. AVCHD formats are "computer-like"... cameras can choose various different bitrates, based on a trade-off between quality and storage requirements.
As far as real formats go, HDV has been the quality winner for quite some time. This is largely based on the fact that MPEG-2 was a fully mature video CODEC before the first HDV camcorder shipped. So there's been a bit of improvement in sensor tech over the years, but the format has pretty much been good from the get-go. AVCHD has been evolving. In theory, it should offer at least twice the coding efficiency of MPEG-2... same quality at half the storage space. And that'll probably be true in 5 years, but it's not quite there yet, particularly when you're encoding in realtime on a camcorder. To offset this, newer models have bitrates up to 24Mb/s, and they're starting to match and even sometimes exceed the quality of HDV, at least in come circumstances.
Format is also an issue. I never recommend DVD-based camcorders, simply because the DVD is too small.. 1.4GB per layer, versus 12-16GB on a mini-DV tape. DVD camcorders record at lower bitrates to stretch this space, and even at that, you get only about 20 minutes per layer (at the top quality setting), and that's lower quality than you would see with HDV or other AVCHD models. Hitachi also makes a Blu-Ray camcorder, which seems like a good idea... a single-layer 3" Blu-Ray disc stores 7.5GB, enough for about an hour's worth of video. I own one of these... it was probably worth what I paid (heavy discount), but the quality is poor compared to HDV except under ideal conditions (I own two HDV models, too).
Hard disc drive had the advantage of long recording times, so if recording beyond an hour without stop is important, this is an option. Most HD HDD camcorders are AVCHD, but HDDs are fast enough and large enough that they don't need to make the same compromises they do for DVD. The main problems with HDD: they are the most fragile sort of media, and once they're filled, you need a PC to unload video before you can shoot more (or you have to delete something).
Flash is my "computer-friendly" format of choice. I've seen fast 8GB SDHC cards for $20... they're of course reusable for years if not decades. A few models have internal storage as well, so you're already able to get >1hr recording time without swapping media.
I'm a big fan of Canon's flash-based HD camcorders.. if I needed another consumer/prosumer-class camcorder today, I would probably buy one of these. Panasonic makes some very interesting pro/prosumer models, but their consumer models are kind of a mixed bag. They pushed for 3CCD sensors in the SD days, and have moved this to HD, which seems like a good thing. Only, as before, they're using three tiny (1/6") sensors together to deliver a not-quite-full-HD image, and they suffer in low-light compared to the large single-chip CMOS sensors used by Canon and Sony. Some of the rationale for 3CCD sensors is lost in the HD models anyway... it was a bit more settled a question in SD days.
You really do need the latest and greatest computer for AVCHD editing. My main PC here, a few years old dual-core Athlon (2.2GHz, 4GB DDR), does a fine job editing HDV, but AVCHD brings it to is knees. It's better on my Quad Core2 machine (2.4GHz, 4GB DDR2, usually "the work computer" running Linux).
Even at that, if I had a large project with AVCHD files, I would probably convert them all to CineForm (an "intermediate CODEC", Apple has one too... these became popular as an editing format in the days before realtime MPEG-2/HD editing was really practical). It's not a big deal, just something to be aware of. Intermediate CODECs generally use a different style of compression (CineForm is based on wavelets, not DCTs), so you don't get "recompression artifacts" messing up your video. The main downside is size: CineForm runs around 50GB/hour for HD. Thus the 4TB of external storage sitting here on my desk...
Technically, though, for a small video in MPEG-2 or a somewhat larger one in AVCHD, it's worthwhile to maintain the original format if you can, if you're authoring for Blu-Ray, as you many not need any recompression (except, of course, for where you edit). You can't get better quality than that!
Edit ability totally depends on what computer you are using. Trust me on this if you don't have a super fast PC 2.8 GHz or an newer apple running Final Cut Pro editing software stay far away from HD, HDV or AVCHD. I have Sony HDV AVCHD camcorder that records to Flash/memory stick for Sony and I can't edit my footage as beautiful as it looks until I buy a $1200 PC or a new Mac. Mini DV is still the easiest to edit if you have average equipment. Quality wise there is no comparison between HD and Mini DV but I feel the Average hardware is not yet equipped to deal with HD format.
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