When it comes to performance and pricing, what is the biggest difference between Dell EMC Unity and IBM FlashSystem?

As with any engineered solution, it depends on your needs.  

However, the bottom line is that for their target markets Dell EMC Unity will generally have a better price at parity performance over IBM's FlashSystem.  

Both are focused on All-Flash arrays and Dell EMC Unity is where I start with VMWare. If I have a dedicated IBM DB2 application, I would lean toward the IBM FlashSystem. 

The problem in the VMWare environment is that IBM has done a poor job prioritizing this area and has several I/O bottlenecks and interface driver issues. I expect future resolution, but does that happen before current platforms evolve?

Depends what you're expecting. Full compatibility with VMware environment - DellEMC only, IBM FS has problems with iSCSI connections to vsphere 7.02 - it's not supported (FS5200 and vsphere 7.02 server with Intel cards - doesn't work fast (10Gbe - 300MB/s instead of 1,1-1,2 GB/s), no solution for now from IBM and VMware (08/2021). Integration - DellEMC and VMware are one company - everything goes smoothly. Space reclamation didn't work well on IBM systems when connected to vsphere (vsphere 6.5 and V7000 models). When using Microsoft virtualization - no difference - it's more complex to start system but when properly configured - it runs well (fast). But of both of them I would choose HPE systems,:-) (Nimble or Primera):-)

EMC's Unity is not comparable to the performance of an IBM FlashSystem option, but it is fairly close in performance profile to the just released Storewize V5100 model 424. If you provide the initial amount of storage you are going to purchase and your expected capacity growth rate, plus any workoad/performance info you have on hand, and we will give you a very quick TCO and performance comparison.i