There are currently two models of the 10.1″ Acer Aspire One being sold in stores — the Acer Aspire One D250 is essentially a slimmer version of the D150 but sacrificed a couple of features. Check the comparison chart below.
The biggest difference between the D150 and the D250 would be the dimensions — 1.31″ thick on the earlier model versus just 1″ on the newer one. Despite that their weight difference is almost insignificant (0.06kg or 0.16 lbs). Maybe they just re-arranged the spacing of the parts inside the chassis.
However, the newer model has a lower rated battery (4400mAh vs. 5200 mAh) and the newer D250 looks like it has more keys (depending on the region perhaps).
The Acer Aspire One D250 is the model with an optional 3G modem although the ones I’ve seen in stores still say it’s just WAN-ready and doesn’t really have the 3G modem installed.
We have a review of the Acer Aspire One D150 here.