We settled on ChannelGrabber after checking out some reviews and they didn't disappoint.
They were really helpful getting us set up. We haven't tried any of the alternatives but I'd be surprised if any of them offered such good customer service. We sell a very large range of products and without this it would have been almost impossible to manage our stock across platforms. The software itself is fairly easy to learn and works better than we expected. We thought we'd have a few issues making the transition but there really weren't any, it just works.