Hello welder5354, about the diameters that I would recommend, consider the application when you are making a selection as to size. The smaller the diameter of the wire, the higher the price typically, if you are welding on a lot of heavy structural(at least 1/2" and up) and you are welding most of your materials in the flat position then definitely use a larger diameter filler wire or as large a diameter as you or your welders are comfortable with. The only times when I would suggest a smaller diameter wire would be when there is considerable out of position welding or when there are issues with welding machine amperage capabilities.
As far as rating wires goes, I would make this suggestion. Most welding and consumable suppliers will be willing to work with you to find a wire type that you will be happy with, so if you don't already have a wire that you are satisfied with see about making arrangements to give some of the others a try. There are many different suppliers of FCAW wires out there and more are coming on the scene all the time, that is one of the reasons that I wouldn't make a hard and fast suggestion, another reason has to do with availability of wires in specific geographical areas, sometimes your local suppliers can give you a really superior price on a specific type of wire and after you have tried that particular wire and possibly compared it to one that someone else has suggested you will find that any differences between the two wouldn't justify the higher price. Don't count out the differences in shielding gases either, sometimes you will have a wire that will perform just fine with the use of CO2 shielding gas for a particular application, if so, go with it, as the cost savings realized by using CO2 compared to 75/25 can be considerable. Along the same lines, consider that many times using 75/25 shielding gas can allow for considerably less clean-up costs and additional deposition rates, better metallurgical weld performance or other bonuses, so don't discount that either. Just a few items for you to consider as I'm sure that others will have suggestions also. Regards, aevald