Being a traditionalist, I'd go for oak or other appropriate 'real' wood.
There are several tricks of the trade to get a good corner joint, but first of all you need to invest in and become best of friends with a 'coping saw' ( seen here:
http://www.lowes.com/...L=1&productId=1018541
Problem is, if the coving is curved or other wise decorated, a simple 45 degree cut using a mitre saw doesn't give a good fit. The coping saw, once mastered, can fix that problem for you. I have coving I've installed after being taught by one of the local Norwegian masters and (after much practice) the seams can hardly be seen. ()I'm in the U.S.).
It's either that or buying a can of 'wood putty'...