I do not have a practical direct comparison. But there are two main points that distinguish these two APs.
1. If your Chromebooks support 802.11ax (WIFI 6), then they can benefit from the new features. Among other things when many smaller data packets have to be distributed to different clients, then WIFI6 brings advantages.
2. The AP250 supports 3x3:3 MU-MIMO the AP305C only 2x2:2, that means if your Chromebooks support 802.11ac MU-MIMO they can use 3 streams. For example, three Chromebooks with 1x1:1 can receive data from AP simultaneously if the requirements for beamforming are met. With the AP305C, this would only be two clients (since 2x2:2).
You can see if your clients support 802.11ac with MU-MIMO in XIQ under Client360. However, you can only see whether they actually use it if you create a trace in the air.
Since future clients will surely support WIFI6 it is a good way to use the new APs, but because of point 2 it might make sense to use a 3x3:3 AP.