It is is a Core i3-7100u (2.4GHz) vs. the Core i5-4200u (1.6GHz / 2.6GHz), then I would just go for the Core i3-7100u. Despite the 200MHz difference, the performance should be nearly the same especially since Intel tend to improve the performance about 5% every generation; so a 7th gen CPU running at 2.4GHz is roughly the equivalent to a 5th gen CPU running at 2.65GHz.
The i3-7100u has better power efficiency so assuming all other parts are equal, a laptop with a i3-7100u may have 30 to 60 minutes longer battery life compared to an i5-4200u with average usage.
The 7th gen Intel HD 620 graphics core is more powerful than the 5th gen Intel HD 4400 graphics core so you get better gaming performance with the i3-7100u. Just keep in mind...