There are a few what-ifs that go along with this. But, the short answer is that sand topdressing is a long-term process and it's not going to add OM to your soil. So, if you have a low-thatch producing turf (fine fescue) and a heavy clay lawn that's giving you problems, core aerating will help you more than topdressing, either with sand or compost. If you *REALLY* need to change the composition of your rootzone (ground is so hard you can't pound a stake into it or it never drains, etc), then maybe you would want to core aerate, then topdress with compost, just to lighten up the soil characteristics in the upper part of the rootzone. Whta I was talking about was regular light sand topdressing applications (done monthly or so) in loamy soils. If you compared that to regular light compost applications, you would get more benefit out of the sand.