As another post said, it's largely that there are alot more 250's out there due to the price point. The 250 is hard to beat for the price. I have both a 250 and a 1500 (and a CZ5). The 1500 is definately more technically advanced than the 250, will save some digging time, and offers a lot more information and features to the operator (size, material indicator while pinpointing, volume control, backlighting, etc).
I recently upgraded to the black epoxy coil on the 1500 (from the white plastic coil). A big improvement, no more "dinging" all of the time when you hit some brush, etc.
Eighty percent of detecting is knowing your machine and how it behaves over different targets (and multiple targets). Re. "target size" - a lot of times the 1500 will show a small target size for a coin for a split second, then jump to a large target size and stay there. Via re-pinpointing a few times in different directions, it will begin to "stick" to the small target size. Deeper coins that have been buried for years, or certain soil conditions will create a "halo" and also look larger than actual size.
Coins near the surface (top 2") will pinpoint "hot" - pinpoint will quickly go from max to zero. As they get deeper, pinpoint off and on will be a little "slower." If something pinpoints high over a large area (6") you know it's large (no matter what the size says). Keep in mind necklaces can be large - if the detector is in that metal range. If the 1500 indicates a large size target, but the pinpoint goes quickly from max to zero, it's probably a small target near the surface.
Keep detecting and digging!
Mike