Err.. The EZ Flash 3in1 is a separate slot 2 card used only for single GBA games, RAM and rumble. So obviously it cannot run POR.
If you're talking about the EZFlash V (slot1) it would be an okay choice since the hybrid patcher would allow you to play games you normally couldn't (eg. Castlevania: POR)
The R4 and M3 are alright though. If you really need to play Castlevania you may have to purchase it if you buy an R4. The R4/M3S are simple to use, but the EZV isn't exactly hard to use either (requires a savelist - this is probably the most irritating thing about it)
EDIT: I'm quite sure about the EZV being able to run Castlevania: POR with a Sandisk 2GB microSD and hybrid mode - but I don't see why it wouldn't.