I am trying to compare the basic strength of two similar amines
Usually this is done by comparing the relative stability of the ion formed on the protonation of nitrogen's lone pair.
This can be done by comparing electronic factors, steric factors, solvation of the ion, etc.
In this example, the stabilisation of the ion due to the +I effect of the groups is the same, as the groups are almost identical.
How will the solvation of the ions formed differ as they have slightly different structures?
Are there other factors that one can consider to help theoretically conclude the relative basicity here?