My father has the 250. I just grabbed a 350 about two weeks ago. I have also noticed a big difference between the two (DD coil plays a big part in their difference). My 350 will false even if I swing and lift from parallel to slightly out from parallel (angled to ground). From what I have been told its somewhat common (annoying but I will get used to it). I hunt with both the 250 and 350 off and on. I feel both are wonderful detectors. I am still learning with the 350 but am getting better with it every time out.
The 250 is usually (90% of the time) dead on while in coin mode. If it is a penny it will hit up penny (occasionally dinging in at a dime). If it is a nickel its dead on nickel. If it rings up quarter its for sure a dang quarter haha. I call the old mans 250 the quarter killer cause he pounds out the quarters out of the ground with that thing.
The 350 from what I have noticed (searching over the same area, sometimes over the same target when we were hunting together) will give crazy signals. I have seen a penny come out of a hole that my 350 'bells' in at a quarter while my fathers 250 rings in at a penny dead on. Again, I am still playing around with the 350 and learning on it but I did find out the last couple times out that lowering the sensitivity has helped out a bit. I left it at the default of I think 5 out of the box I lowered it to 4 bars and it seems to work better for me.
Anyway, I dig both machines. I wish I had saved a few more pennies to snag the at-pro but I think I will get plenty of use out of my 350.
Neil