Then I would suggest going with an iMac without a doubt. The only benefit I would see for the MB Pro 13 is portability, which you’ve said isn’t a concern.
Not only are you getting a much more workable screen for 3D modelling, but looking at the specs for a $1500 21" 4k iMac vs $1500 13" MB Pro you get: 3Ghz 6-core vs 1.4Ghz 4 core, same 8GB RAM, 4GB radeon pro vs the integral intel iris, 1TB Fusion vs 256BG SSD.
I think you’ll find the iMac much more workable, and while an SSD will give you better speeds, you’ll find if you’re working with files up to 0.5GB you’ll fill 256GB pretty quick. The processors and the graphics on the above iMac are easily superior to the 13" MB, and if you can find $200 I would suggest bumping the RAM up to 16GB, if not straight away then in the near future.
If budget is a factor, and you’re happy to look at desktop solutions, you will generally get better specs for your money in windows solutions… And a superior version of Rhino.