Comparing the cheapest flights, EDI-AMS on EZ is around £35-40 while EDI-LHR on BD is around £60-75.
If I am going to stop over in a third city I'd much rather Amsterdam than London anyday!!!
The Scottish Exceutive have made noises about a rail link to EDI in their latest transport plans.
BAA owns both GLA and EDI and have traditionally favoured GLA. However, there are some signs that the airlines prefer EDI and most new services recently have come East.
Stephen