I know this may sound like a daft question, but if i want to create a simple model that can be created in either the Spline or vertex modeler. Which one of these would use the less CPU. The reason I ask is because I am creating a complex scene and need to keep the CPU usage down as low as I can. Any help on this would be greatly appreciated.

Guessing spline. Maybe experiment. Model an example in spline, then copy and paste it ( not duplicate it) and convert one instance to mesh. Then compare the two sizes eg when you save them. That might shed some light, maybe not. Doesn't take into account eg uv texturing etc. 

sort of depends, spline is actually worse using the Octane render plugin as it converts it to triangulated mesh anyway and is often massive as very smoothed

not to mention you need to use a suitable UVW projection function to texture it

most people are not using Octane though to be fair

I did try the earlier suggestion of converting it into a vertex model and then I checked out the result. A massive YUK! It had turned it into nothing but a triangulated mash of which there was no way I would model like that "I always use quads in my modeling" and so I knew the vertex model would show as a larger model than the spline. As far as rendering goes, have never heard of the "Octane plugin".. what is it and where do i get it from?

Otoy but it's subscription only now

I bought it almost a decade ago, you need a Nvidia card and not sure what's supported now, I know my older version OR4 will not support cards much later than my 2080Ti