In case anyone is interested I did compare cpu and gpu rendering on my 3 machines. I used the test scene which I believe is now in free stuff here (Pauline in the P.I.C.K room). I changed the rendering from progressive to normal (I find watching the individual buckets render helps me to decide if I've chosen wrong bucket size). For gpu renders I used 512 bucket size and for cpu 16. The results are probably best not read across from one machine to another but you might find interesting.

Machine 1

Gpu GTX750M 2Gb time 3 hrs 18min cpu I7 4770S 4 core 8 thread 3.1GHz time 1hr 35m. Yes GPU takes over twice as long as cpu

Machine 2

gpu gtx980M 6GB time 36m 11s cpu I7 6820HK 4core 8thread 2.7GHz time 1hr 43m This is a gaming laptop and as expected the gpu is much faster than the cpu

Machine 3

gpu gtx1080 8Gb time 21m 20s cpu I9 7940X 14 core 28thread 3.1 GHz time 25m 05s So although gpu is faster, not a huge difference.

So if I was looking to upgrade these days, not knowing when or if poser will work with rtx cards for gpu rendering, and not knowing if it will ever take advantage of rtx features, I'd propably opt for a budget gpu and spend more money on a top end AMD threadripper or similar.