I am by no means a Camcorder expert, but I'll try to answer the best I can. Hopefully someone with better technical knowledge can correct any mistakes.
First, let's disambiguate the terms HD and HDD so that no one gets confused. In the camcorder world HD is coming to mean High Definition (either 720 or 1080 lines of vertical resolution compared) while HDD is coming to mean Hard Disk Drive.
The next thing to clarify is that mini-DV and hard disks are storage mechanisms and not formats. You could write using the same format, compression schemes, and encoding schemes to both media. In fact mini-DV and Sony's Digital8 both use the DV video format. It's not the storage mechanisms that makes a difference in editing (after all, you have presumably copied the data onto the computer before you started editing); it's the format.
The most common formats used on mini-DV tape are DV (for standard definition recording), HDV (for high definition), AVCHD or H.264 (for high definition). DV uses a compression technique in which each frame (one picture in the video) is compressed all by itself. HDV uses mpeg2 compression and AVCHD uses mpeg4 compression. These compression techniques use a key frame and then the next several frames rely on information from the key to decompress them.
Let me try to make that a bit more clear. If I shoot 1 second of video using a DV camcorder at 30 frames per second, I get 30 frames. Each frame is stored with all of the information needed to display that frame. If I shoot the same 1 second using mpeg2 or mpeg4, I might get 1 key frame followed by 14 other frames and then another key frame and another 14 other frames. To display frame 8 (just to pick one at random), I need the key frame and the 8th frame. Mpeg2 and Mpeg4 can fit much more information on the same amount of tape (which is why they are so popular for HD). The downside, as you noted, is that they are harded to edit.
There are two reasons why they are harder to edit. First, there is that key frame/non-key frame business. The computer has to look at more frames to display just one frame. Second, the compression rate is higher and requires more computations to decompress. In other words, the computer needs to look at more stuff and do more work on it just to show one frame.
As far as I know, every hard disk based camcorder uses a form of either mpeg2 or mpeg4 compression. So in that sense, video recorded from a mini-DV camcorder in standard definition will be easier to edit. On the flip side, it'll take up a lot more space on your hard drive while you are doing all of that editing.
One thing that is likely to improve is that your future computer will probably have special chips for decompressing mpeg2 and mpeg4 video streams. That will make reduce the performance difference considerably.
As for tapes being on the way out, I think I mentioned that. I'm not sure whether they are dying or not, but they are certainly going from being the overwhelming choice of camcorder makes to being one of many choices. No one at the Consumer Electronics show seems to be putting serious effort into improving standard definition mini-DV camcorders and some manufacturers are shrinking their model lines. The situation may stabilize so that there are mini-DV camcoders, HDD camcorders, and flash card camcorders. On the other hand, it may be the start of a trend where tape fades away.
There are a lot of worthwhile differences between tapes, hard disk drives, and flash cards. Aside from formats, there is convenience, cost, reliability, durability, longevity, and compatibility concerns. The great thing about having options is that you have a lot of choices. The bad thing is that you have to make a choice and that sometimes requires an annoyingly large amount of research.
Personally, I'm holding out for a high def camcorder. I love the Canon XH A1, but I'm nervous about HDV as a standard. There is also the small matter of it being shockingly expensive. I'll keep waiting.
