Both games are probably in the top ten best PSP games no doubt.
This is probably the best analytical comparison review on Peace Walker HD since you seem to be comparing it to Birth by Sleep.
The problem with Peace Walker HD was not the updated aesthetics, but the fact that it was a PSP game being ported to the 360 and PS3. It still played and felt like a PSP game regardless because the game was originally developed on a system with far less superior hardware than the 360 and PS3. The AI, for example, in Peace Walker is extremely limited and not many changes were included in the HD port.
Birth by Sleep will probably experience the same fate. No matter the graphical overhaul, it will need vast changes to make it feel more like a console game than a PSP port. After reviewing the changes in Kingdom Hearts 1.5 ReMix, this seems to be more and more plausible. Kingdom Hearts 1.5 major changes only consisted of modern controls, HD textures, and....a extra slot. In some sense, Peace Walker HD received similar changes.
If you are worried about aesthetics, then fear not. The poly count and resolution for Kingdom Hearts 1.5 seems to be on a whole new level than Peace Walker HD. It could be assumed that Birth by Sleep HD will look better than Peace Walker HD, but the game play will most likely, inevitably stay the same.