Do not listen to people saying it's always one way or the other. It's not. It depends on the item and your class/spec. 10 item levels vs a socket is not always going to be an automatic socket choice since the items scale differently as they increase in ilvl. As Roboartist mentioned there are multiple variables involved and it can be a tricky decision to make.
One thing to consider, as the item levels increase, the primary stat of many BoEs increases faster. This creates a curved graph instead of a straight line for primary stat in relation to item level. Example:
Aethrynn's Everwarm Chestplate
835 = 1184 str, 1777 stam, 509 crit, 787 haste (old base ilvl)
840 = 1241 str, 1861 stam, 519 crit, 802 haste (+57 str, 84 stam, 10 crit, 15 haste)
845 = 1299 str, 1949 stam, 529 crit, 816 haste (+58 str, 88 stam, 10 crit, 14 haste)
850 = 1362 str, 2042 stam, 538 crit, 832 haste (+63 str, 93 stam, 9 crit, 16 haste)
855 = 1427 str, 2140 stam, 548 crit, 848 haste (+65 str, 98 stam, 10 crit, 16 haste)
860 = 1495 str, 2242 stam, 559 crit, 864 haste (+68 str, 102 stam, 11 crit, 16 haste)
865 = 1566 str, 2349 stam, 569 crit, 880 haste (+71 str, 107 stam, 10 crit, 16 haste)
870 = 1641 str, 2461 stam, 580 crit, 897 haste (+75 str, 112 stam, 11 crit, 17 haste)
...continues to slope upward...
Since str is king for my DK, choosing between an 860 socket vs 870 non socket chest piece for me would be the 870 since 146 str, 219 stam, 21 crit, and 33 haste is better than a 150 secondary stat gem. If I had no sockets at all then maybe a 200 str gem would be better, but I know I'll eventually get the legendary armor piece which has 3 sockets so that is automatically a moot point.
Some classes such as my BM hunter prefer a secondary stat over a primary stat so the circumstances have change and 10 ilvls may not be a better choice of a socket.
