Rhythm is something you can develop. Listen to some of your favorite songs...preferably not thrash...and try to follow the drumline...I mean, just listen and hone in on the beat. Before you know it, it will transfer to your playing, since that is what you are training yourself to do.
As for which instrument? Which sound you enjoy producing? To me, metal guitar is like someone who talks a lot but doesn't always have something of value to say. When the bass talks though...people listen
