There are methods for doing KLM - GOMS evaluation and the like, but this would typically be used for comparing two designs for the same goals rather than comparing two products, which even if similar in features will most likely be quite different in actual usage.
So if you're trying to compare products, the best start would be to break it down into chunks. Then compare the similar chunks, and contrast the differing chunks.
Here's some suggestions as to how you might approach this, by looking at roles and goals, but you'll need to adapt this to suit your situation.
Looking at your existing tool:
- List the features of your existing tool
- List the roles of your users
- List the tasks you currently undertake, and to what user roles they apply
- List the attributes that can be assigned to those tasks - frequent, onerous, repetitive, secure.
- List the ease or difficulty of the tasks (multiple steps, cognitive load, etc)
- Extend this list as appropriate for your scenario
Then looking at each of the new tools:
- List the features of the new tool
- Look at how each task would be achieved
- Assess whether each of the tasks will retain the same attributes, or whether it will become easier or harder, longer or shorter, etc
- Combine those results to determine whether each user role will become easier or harder
- Assess what added value the new features can provide and rate that value (eg: Won't Use to Will Improve Productivity)
- Assess what functionality will be lost by moving away from the existing tool and rate that loss (eg: Not An Issue to Can't Do Without)
Hopefully, you'll start to see that the new tools either compare favourably - or not. And maybe you'll notice that the new tools either lack essential features, or provide opportunities to improve efficiency, either directly, or in the future.