Although I have spared no expense in bashing the SPG50 in the past, I do have to admit that I twice saved some money over the best available rate at the Essex House (not much, about $20-30). Of course, there were plenty of other times where an SPG50 rate was not available or not worth it.
Also, I realize that this is an already-discussed topic, but the Essex House will be leaving SPG soon. Might be a good time to give them a try and see if you can offload a 50.