The Ace 350 is a little higher frequency, so it has better iron discrimination. All other features (modes, notching, etc.) are the same. So for the $85 difference from the 250 to the 350, you get the bigger coil, DD coil benefits of better target separation, a little better iron discrimination and a set of headphones.
Ace 250 6.5 KHZ / Ace 350 8.25 KHZ / At Pro 18KHZ
The lower KHZ is a little more conductive to silver and iron, higher is more conductive to gold. Detectors from various manufacturers run from 3-75 KHZ. Even though the At is a higher frequency, it is still very sensitive to silver.
If it were me, I'd try to move up to the At Pro, it's a great value for the money. Of course, I'm biased (dealer), but we get a lot of people that start with an Ace and move up to the AT series, some within just a few months.