Jon,
I can only speak for GMG. I have done several installs of GMG ColorServer and the product is incredible for color management. When iterating a device like an Epson X880 or HP Z series, you can expect a delta e average less than 0.50 with very few colors above 2.0. Of course, that depends on what stock you proof on. If you use GMG stock, you will get these numbers easily on semi-matte. I am sure the same can be said of EFI as they are both great products. Everything depends on the ease of use. GMG can be installed in parts depending on what type on installation you want. What I mean is if you want the ability to make your own color profiles, you need Profile Editor which comes at an additional cost. GMG will function perfectly fine without this tool, your installer will make the profiles for you and you will still have the ability to calibrate the device through the GMG wizard. Do you have any offline devices like an iGen that you would like to color manage? You can use GMG ColorServer for this as well to convert a PDF into a color managed PDF for that workflow. Please note that PMS colors are passed through this workflow and are not color managed. Are you located in the US and if so, what market are you in? I have done installs all over the midwest and Seattle and NYC so I could get you in contact with someone to give you a demo. I am a certified installer of the product, you can call me at 651-334-9203 for more info. NO, I am not in sales for the product and am not affiliated with GMG.
Mark