The first computer that comes to mind for an Apple desktop for best performance for less than $3500 is the iMac 5K with lots of upgrades, but the Mac mini gives it a run for its money. AppleInsider details two high-end options for the iMac 5K and Mac mini, and compares them head-to-head.
Source: macinsider

Mac desktop shootout — $3500 iMac 5K versus $1700 Mac mini
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