Radiant Dawn is great from a gameplay perspective, and pretty much terrible in everything else.
I don't really want to start a flamewar over tastes, here, but... it's friggin' FIRE EMBLEM. What are you playing it for if not gameplay? All FEs are pretty bad for story/graphics/etc. anyway (besides character art which RD does just as well as any other game).
As you can probably gues I consider RD superior, but I can see why someone would prefer PoR. PoR is easier and much more straightforward (you can just choose one team and run with it). Regardless, RD is absolutely worth a try to see what you think about it!
Regarding Calill, I'd definitely use her if either of your first two mages aren't looking too good, her stats are solid and she can use every long-ranged tome. I'm the opposite of one of the previous posters here, I usually use a real healer + Calill because I lack the patience to spam Heal staff use on a combat character until they can actually use the staves I care about (Physic and Restore). Still, your milage may vary there. You have one more Sage left to get, and sadly he is terrible.
--
The RPG Duelling League: www.rpgdl.com
An unparalleled source for RPG information and discussion