For long CPU intensive work the difference is probably small, like < 10%. The change between 8th and 9th generation i5 seems to be small and down to the clock speed. Speaking of clock speed, the top model has both higher base frequency and Turbo Boost, which *might* make the whole system feel more snappy (personally, I care much more about snappiness in everyday tasks than some +/- for asynchronous tasks like rendering).
570 is not bad, the 580 is better, but still not the greatest of all time. If you do graphics intensive stuff or use an external display, that should go into consideration as well.
I think the saving is just too little to go with the smaller model (and when you sell it - why should anyone bother to buy a used base model if there will probably be plenty of top models around).
As for the SSD - how much are you currently using? Are you willing to spend money on an external drive (slower but probably fast enough or really fast and quite expensive)? This comes with additional costs that is often overlooked:
- it always needs to be connected sitting on your desk (or fixed on the backside)
- managing data on two volumes may be more troublesome than on one drive
- one more drive means one more point of failure
It does come with a couple of positive sides as well...
- iMac dies => some data still available
- drive easy to plug into another machine
Given the latest price reduction on the SSDs by Apple, I went "all in" there as I hope for hassle-free and it was finally justifiable (would have chosen a smaller one the day before the reduction).
So as always it comes down to what you are doing with the machine. You can talk yourself right into the maxed out i9/Vega48/2TB/128GB, or get the base model and be happy.
Just make sure not to pay for the RAM but get some 3rd party sticks.
