The EJ123 is a battery powered SCALE with a Load Cell at its core. The FX-120i is an AC powered BALANCE with an electromagnetic force restoration BALANCE at its core. In principle, a BALANCE is less prone to drift than a load cell based SCALE, and calibration is more reliable and incrementally scalable for BALANCES, moreso than load cells.
Serial port controls, memory functions, etc - aka, a lot of stuff which reloaders don't use, except those of use using the SuperTrickler or AutoTrickler equipment - are other feature differentials. For what you're doing, assuming you can confirm speed and drift reliability (which I honestly expect to be better than, say, the scale used inside an RCBS Chargemaster), AND confirm there is no measurement lockout (which I assume there is not), then I'd bet on it serving you well. I used relatively similar Mettler Toledo models in some mobile labs I ran back in 2010-2017; we had an entire fleet of fixed asset Mettler Toledo analytical balances, but we were breaking down and setting up $4000 machines sometimes multiple times per day on multiple locations, so I converted my "firefighters" to use this kind of battery powered unit. They worked fine, and were a quarter of the price of our standard lab balances. We did have some lesser units, sub-$200, which I can't say I recommend, but I think you're approaching a relatively safe price point, from a relatively well proven brand.
