the 350 has the 8.5 x 11 double d coil
the 250 has a 6.5 x 9 concentric
the 350 runs on 8.25 kHz frequency
the 250 runs on 6.5 kHz frequency
everything else is virtually the same. Haven't really got to use them yet, got home late from our grandson's 5th birthday party; but have an "expedition" planned for tomorrow.
dont buy one, buy a fisher f2 for 215.00 and you get a free 4 inch coil and a free pinpointer, i just sold my ace 250 never do i want to hear that stupid belltone again.