Looking for advice on software escrow services vendors

Does anyone know of a good review of source code escrow services in terms of features, costs, key terms of agreements, etc.? I'm looking at a few including:

(a) Iron Mountain. http://www.ironmountain.com/Escrow (b) InnovaSafe. http://www.innovasafe.com/SourceCodeEscrow (c) Escrow Associates. http://www.escrowassociates.com/Software

My specific use case is to periodically provide source code uploads during the 9-12 month development process of a custom application for a single client.