Definitely the ST250DM000. It's a newer model than the other one.
ST250DM000 has one disk and read/write head(specifications):http://www.seagate.com/docs/pdf/datasheet/disc/barracuda-data-sheet-ds1737.1-1111us.pdf
ST3250318AS has two disks and three read/write heads(specifications):http://www.seagate.com/docs/pdf/datasheet/disc/ds_barracuda_7200_10.pdf
More disks and read/write heads are worse. That means density is lower thus it's slower, uses more power per GB, it's less reliable, and it's heavier.