They both use the same MiniDV tape, The rate of compression varies depending upon the format recording in and thus the amout of time available to record.

Pros: HD has Outstanding Visual Sharpness, SD/DV has Also great picture quality, better than SD Television broadcast. DV format is very easy to edit.

Cons: HDV(High Def Video format), Currently Is Very Hard to edit because the resources required to process the amount of information is needed. If you have a old computer stay away or upgrade.

AVHCD is the new mp4 High def codec(found in the new Hard drive cameras), There is little real world support for it now, i’d Stay away until next summer when more programs(and support) are out for it. DVD, There have been advances in Recordable dvd in camcorders in the past few years, it is a very good format, but editing it is a pain, the dvd .vob(the Visual OBject file wrapper, which these cameras record in) wasn’t meant to be edited and the editing programs now have a hard time with is format, since it takes a while to process(and reprocess) to get the end result(a lot of time)
Advice: Your choices come down to two, money and quality. Most DV cameras cost around $300, which will give you great quality movies.
HDV(High Def Video) starts around $800-$53,000, yes that much!)Which gives outstanding video, but at the high cost of the camera, upgraded editing system and the required HD Television to get the required resolution for the HD content, your looking at a minumum upgraded cost of $5000 for HD + another $1000 for a high def DVD recorder which has not been released yet and the HD discs start off around $50 each.
So my Best advice for a nOOb is to start slow in DV and upgrade after a few years when the technology is more ripe and you can save up for the better equipment. a DV camera will cost you $300, + a Firewire card $70, + DV tapes $2 each, Firewire cable $2-3. Compared to the $5000 cost of HD. easy choice for a beginner. and if your wondering about the quality of DV, Don’t. Dv is better than any analog camera, better than VHS and Hi-8 and ALL TV stations use this dv format for their Handheld cameras, since it is cost effective and high quality.