As Aela said, vote how you feel. My preference is Radiant Dawn, for the following reasons:
No map animations. This increases the pace of the game so significantly that Radiant Dawn is only 30 minutes (or ~20%) longer to beat despite having 40% more chapters and significantly more combat (42 vs 30).
Constantly switching armies forces you to use more units and employ a little more strategy than soloing the game with an overpowered flier. Granted, Haar does handle a huge portion of the game on his own, but you're still using Edward, Nolan, Sothe, Tauroneo, Jill, Zihark, Muarim, Elincia, Lucia, Mordecai, Geoffrey, Kieran, Ike, Titania, Oscar, Janaff, Tanith, Sigrun and all the laguz royals. Compare that to Path of Radiance where you're almost exclusively using Ike, Titania and Marcia, and occasionally Tanith.
The map objectives in RD are more varied and innovative than in POR. Almost all the maps in POR are seize, whereas in RD you're doing things like burning supplies, having Ike and Ranulf simultaneously arrive, and defeating 4 bosses in a row in endgame.
The bonus exp mechanic in Radiant Dawn is more favorable to speedrunning, especially since Path of Radiance speedruns used fixed mode growths. Combining transfers with BEXP guarantees that Ike (tier 2), Haar, Titania and Jill are always maxed out, whereas POR BEXP leads to an average stat Marcia that often risks death late into the run. (editor's note: even if Jill does get RNG screwed in RD, it only costs a few minutes at most)
The skills system in Radiant Dawn is much better (since you can re-assign them) and further contributes to its strategic complexity compared to POR, especially the pass skill.
There are a couple other minor points but that's the general gist of why I find Radiant Dawn a much more exciting speedgame than Path of Radiance.
