The Oplitplex range is mainly designed for a networking....they tend to change the spec on the product less often, meaning the various PC on the network are more compatible.
The Dimension range thens to be the latest technology & mainly designed for stand alone or smaller networks.
So depends on you need, but chances are you're a Dimension customer!!
My understanding (from the IT guys in work) is that the Optiplex machines are largely supplied to Dell business customers and therefore also tend to be slightly more durable in terms of overall build quality than the Dimension models sold into the home market.
They aren't more durable just more stable in technology terms....the Dimension range is designed to be able to intergrate the latest technology as soon as possible. If you are an IT mgr managing a networks with 100's of PC, you want them to be a similar as possible.....the Optiplex range makes maintenance and upgrading much easier.