7.62 x 39, aka the russian m43 (1943) is a medium power round designed specifically for semi-automatic/selective fire assault weapons. it is effective in the sub-300 meter zone in the siminov sks45, kalashnikov ak47 and akm, but also used as a suppressive fire round out to 800m in the kalashnikov rpk and degtyarev rpd.
7.62 x 51 (.308 nato) is nearly a full power rifle round designed to be used in what are commonly referred to as 'battle rifles'. these would be semi-automatic and selective fire rifles such as the ar10, fn-fal, h&k g3/91, cetme. it is also used in light/g.p. machineguns of various description. it carries more energy to longer distances than the m43 round. in sniping applications it is useful to approximately 800 meters.
both rounds are fine for the purposes for which they were created.
m43 is dirt cheap, about $75 to $80 per 1000 rounds in my area. the firearms that use that round range from the $150 siminov to the $350 kalashnikov to the $2000 rpd.
.308 nato is more costly to shoot, althought the $350 cetmes and cheap fal/g3 clones from century can be great blasting fun. rifles get exponentially more pricey, though, as one goes up the quality ladder, with dsa built fal clones bring $1000 and up...way up.
as the old ar15.com saying goes...GET BOTH!