The E100H and the DVR-510H are very comparable. The E100H has a larger hard drive, MPEG4 support, SD and PC card support. The Pioneer doesn't support MPEG4, SD or PC card. The Pioneer has DV "OUTPUT" support, DVD back-up, DVD dubbing, and records an anamorphic 16:9 with the correct flagging to a DVD-R if the original is flagged(judging by how the manual reads), where the E100 doesn't have any of those features. The Pioneer has "auto start" which turns the recorder on by signal sensing and starts recording when a Digital cable set top box or satellite receiver turns on for a timer recording using the DBS/cable box's "own" timer. Of course, when the DBS set top box turns off, the signal is gone and the Pioneer senses that and stops recording and turns itself off. I found this feature much more reliable and useful than any IR blaster(I've used quite a few.

). BTW, the 510H is also cheaper by a few hundred bucks.
Performance wise, let's say equal. I did find my old E80H to add digital artifacts where I see none in the DVR-510(both in SP and Fine/XP modes). The E80H could had been defective but I doubt it, I think the TBC circuit or ADCs were adding the artifacts as they were there on pass-through while not even recording.
But for the sake of conversation, let's say both have equal performance. Being the different features one has over the other will be user preference and opinion. I prefer the DVR-510's features.
The real deciding factor is what rewritable format do you want: DVD-RAM or DVD-RW. Panasonic only burns to the DVD-RAM and not DVD-RW, while the Pioneer only burns to DVD-RW and not DVD-RAM. Personally, I don't want DVD-RAM because it's an incompatible format that won't play on standard DVD Players "UNLESS" they are Panasonic's upper models. DVD-RW plays on many players. In fact my OLD Panasonic DVD-CV52 that states it can't play DVD-RW, plays them great with no problem. It's played DVD-RWs made on a DVR-310H and a DVR-510H. Also DVD-RW is a well established PC format that's compatible with most PC DVD-Rom drives. Most PC drives are incompatible with DVD-RAM while a select few are compatible.
So for me, the features I want are on the 510H and it's cheaper to boot. Also, the format that's more compatible and looks to be the winner of the rewritable format is DVD-RW, not DVD-RAM IMO. So the DVR-510H is what I consider the better "FOR ME".
Overall, I consider them equals with only personal preference dictating which is better for the user.
If you'll take a loss on the E100H, it may be better to keep even if you like the 510H's features better.
Have a good one.