The following information and opinion on the Ace 350 is based on internet research only. I have never tried one, but I can tell you why I chose the Ace 250 over the 350.
Three reasons... Price, discrimination resolution, and the fact that I couldn't see any benifits to the 350.
.
If you like having more coil options (like I do), you can get the Ace 250 with the stock 6.5 x 9" concentric coil & 8.5 x 11" DD coil for about $330. Ace 350 with the stock 8.5 x 11" DD is $297. So, for about $33 I wind up with an extra ($75) coil. As far as I can tell, the differences between the 250 and the 350 (other than the coil it comes with) is minimal. 2Khz isn't much of a difference in operating frequency. When using the DD coil, depth is about the same.
I know you said that the price wasn't a problem with you, but was for me.
You are correct... Both machines have 12 target ID cursor segments. On the 250, two of the segments are for iron discrimination and 10 for non-ferrous, and on the 350, four for iron, and 8 for non-ferrous. Having more segments devoted to iron may be a benefit for a hard-core relic hunter, but for me, it'd be a hindrance. I very rarely dig any ferrous target on purpose. On the 350, all the coins (other than the nickel) are piled into the top three notches. It can't discriminate between a dime and a quarter... They both occupy the same notch. The penny only has one notch as well. If a signal bounces between a dime and a quarter on a 250, as far as I've seen it's usually neither. You wouldn't be able to see that discrepancy on the 350...
I look at it like this:
Red is iron areas, gold is gold areas, and green is coins and silver...
Sloped areas are overlaps.
Ace 250:
Iron-2
Gold-6
Coins-7 (inc. nickel)
Ace 350:
Iron-4
Gold-6
Coins-4 (inc. nickel)
The one thing I was not prepared for (and it's the same in both the 250 and 350) was the 'processing delay'. It takes about an eighth of a second to process the signal from the coil through the circuit to the speaker and display. As long as you can get used to that, the Ace line is a great choice. For the type of hunting I do, the 250 w/8.5 x 11"DD is the better of the two.
For coins, I think you'd be best off with the Ace 250. Better resolution.
Again, this is all just opinion.