The E6430 has a metal case and is more robust, but uses an older CPU and chipset.
The E5440 is a plainer chassis with a newer generation CPU and chipset.
The E6440 is the concurrent update to the E5440.
There are too many possible configurations to definitively say which is better -- it depends on the CPU, GPU, etc. in the specific model (or that you choose when you order one, but since the E6430 isn't a current model, you may be looking at used systems).
Newer isn't always better -- you have to do a side by side comparison. Sometimes newer isn't better -- if buying a used car, a 5-year old Camry is a better bet than a year old Chrysler.
They're pretty evenly matched (except for the i3) for what you want to do; buy by price and which you like the looks of better. If this is a used system, ensure the seller guarantees no passwords on the system and an unactivated TPM - otherwise you may well find yourself in bad shape.