You might check out the following review, which includes a GemPro500 and a very nice Sartorious GD503 magnetic force restoration balance, somewhat analogous to the two balances you're comparing:
Although this article is about 10 years old, the comparison between the three is still quite valid. The FX120i is a MFR balance. Comparing the FX120i balance to the GemPro250 is not really a fair comparison. Relatively inexpensive gem/jewelry balances certainly can work well for weighing powder and therefore have an established place in many reloading rooms. That is not the same as saying they are on equal par with a precision MFR balance. Not even close. I would suggest buying the
best balance you can afford without breaking the bank, whichever that may be, or at least the
best balance that you can afford without having to forego buying some other very important piece of reloading equipment or reloading components, is usually the best approach.
FWIW - a while back, based on a review from Boyd Allen, a fellow shooter here at A.S., I purchased a little Gem 20 scale. Here are links to a couple reviews of this scale:
Some time back in a thread about scales, one poster mentioned a scale, almost apologetically, I think because it is so inexpensive. His post seemed to indicate that he was an intelligent and experienced person, so I took a very small chance, and ordered one. Boy, was he right. I am delighted...
forum.accurateshooter.com
These scales have been mentioned on this forum before as being a good buy. I noticed Amazon has gotten some back In stock at a good price. Might make a good Christmas present.https://www.amazon.com/dp/B00ESHDGOI/?tag=accuratescom-20
forum.accurateshooter.com
I and others have put this little scale through its paces and the consensus is that it works reasonably well. The key with the Gem 20 is that it only costs about $20. And yes, it works. And it's worth repeating that it only cost about $20. I figured it was worth $20 bucks just to see what this little scale was all about, even if it turned out to be total junk, which in actuality it did not. So now I have a dirt cheap backup balance that I would most definitely use to load for a match if my MFR analytical balance ever failed for any reason. Having said that, I would not choose to have the Gem 20 as my one and only sole means of weighing powder. If I decided to go that route, I'd have at least two or three of them, in case one failed right before an important shooting match, and I'd be darn certain they all calibrated exactly the same.
Don't get me wrong, I'm not at all advocating that you consider the Gem 20 as an alternative. I'm merely pointing out that even a $20 little jewelry scale can do a decent and acceptable job of weighing powder under the right conditions. However, they types of scales will generally not measure up to a [much] more expensive MFR balance in terms of longevity, linearity, speed, precision, re-zeroing,...the list goes on and I think you get my meaning. If you can afford the fx120i, I doubt you will be disappointed over the long run. Will it make a noticeable difference in match scores or group sizes on day 1, as compared to the GemPro 250? Probably not. The readability of the two balances are the same (i.e. +/- 1 mg), if not their actual precision. There are certainly many competitors that have used the GemPro 250 successfully. So if the price tag of the FX120i is just too painful, it's not going to kill you to go with the lower priced balance by any means. I'm simply suggesting that you consider the opportunity cost as means of helping you make a choice. What you might have to give up (if anything) in order to buy the more expensive balance is possibly the best indicator of whether it is worth it.