Hi,
I've been a long time Mac user. I'm still using 2nd gen iMac specially for sketchup modelling full time.
No problem in modelling. For rendering I'm use Thea, V-ray Next all work 100 percent.
Time has been moving fast ,with the new real time rendering applications around. My iMac can't handle those
high computing applications.
You should thing about this and if you are going into realtime in the future i.e
new v-ray realtime, Twinmotion to Unreal Engine or D5 etc.
For me I bought a Window laptop GPU of course just for Realtime rendering proposes.
and do my 3d modelling-small scene rendering projects in iMac 27" .
Anyway I hope that helps in your decision.Cheers