Just got ours today, the wife 250, mine 350....
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.
Todd