An engine monitor will pay for itself. I have used a Silverleaf VMSpc for about 10 years. When I bought a new MH it was one of the first things I installed. Comparing it to Scangauge D I would say the difference is that VMSpc uses a laptop and Scangauge D does not. The laptop provides a large screen and allows logging of data. I have my laptop handy anyway because I use it for GPS. I can vouch for VMSpc and their excellent customer support. Scangauge D looks good from their online description. I would not decide based on cost because an engine monitor will pay for itself many times over.