I use half butter half shortening and I use 5 cups, sifted before you measure, icing sugar, 1- 1 1/2 tsp. pure vanilla, 2 tbsp. unwhipped whipping cream and then thin with whole milk. I found by adding the extra cup of icing sugar, that the icing was less affected by the heat of my hands. I switched to part cream and part whole milk, because I found the icing incorporated more air with using all whipping cream. If you are switching to half butter than you will likely find that using whole milk or cream or a mixture of both, will give you a much better result than using water as your liquid. Aways beat a recipe like this on low power, starting by softening your butter on low before adding your shortening. This gives you a better result.
Some of you may find that it is easier to make your leaves and roses with the all shortening recipe than it is with the recipe using half butter.
Regarding salted versus unsalted, well generally salted butter has a longer shelf life, which is likely why Wilton says that icing made with the butter and cream or milk, is good at room temperature for 2-3 days, but actually they give the same time frame for all shortening icing.
The 2-3 days, though, is generally the recommended length of time to keep salted butter at a room temperature of less than 75F and is likely what they base the estimate on.
Generally, you do get a longer room temperature shelf life with the salted butter because the salt works in addition to the sugar, as the preservative. I find it is more like 5-6 days. The icing can be refridgerated, according to Wilton, for up to 2 weeks, again make sure that the best before dates for your milk, cream and butter, are well within this timeframe. I generally prefer to have the icing refridgerated for up to a maximum of 1 week before using it to ice a cake.
I have never used margarine in icing but I do use it in some cookies, but I only cook with hard margarine, the kind that comes in squares as the moisture content of tub margarine is not meant for baking and such. Generally, you should not subsitute margarine in your baking unless the recipes say you can. Why? Well margarine is affected by oven temperatures and generally was not designed for baking, especially for longer periods of time. It has to do with the effect of the heat on the break down of margarine and what happens to this margarine in the chemical relationship to the other ingredients.
Here in Canada, commercial bakers can purchase specific Baker's margarines that are each developped for a different baking purpose.
Regular domestic margarine was developped as a cheaper alternative to butter, during the depression. That is why many older cookbooks call for margarine.
I grew up with both and find that your tastebuds adjust once you get used to either one. When you are used to margarine, well butter can taste sour. When you get used to butter, well margarines can have a funny sweet taste. When you are used to unsalted butter, salted will taste noticably salty and vice versa.
One warning when switching to a icing that uses half butter, is that the melting temperature is much lower than that of shortening so this may indeed be a factore when you are making a cake that will be outside or inside on a hot day.
Hugs Squirrelly Cakes