Agree with Mostar. Unless things have changed dramatically under reconstruction since the war, Bosnia's road system is fairly slow-going because the country is very mountainous. It therefore limits the distances you can do - you don't presumably want to be in a vehicle all day.
The main attractiveness of Croatia for most people is the coastal split - the limestone scenery backdrop against the blue of the sea. Visiting islands is fine - some of these islands very large large and provide the opportunity for a few days of them.
The coastal road is quite good now and there is a motorway from Split to Zagreb (about 150 miles) that cuts across the high land and runs very close to the N Bosnian border.
When you say N. Croatia, I guess you mean the area around Pula - the hinterland towards Zagreb is relatively land and (IMHO) less interesting.