With only about a £1/metre difference between the cheapest and premium vinyl you are only talking about 15 - 25p saving per shirt, for a large size print, way less for a logo. Unless you are doing a massive amount of single use shirts you have to ask the question if it is worth having £100 worth of cheaper vinyl sitting on the shelf just to save pennies per shirt.
I use mainly MDP Hot Flex for everything. Not because it is any better than anything else - purely so that I now that everything on the shelf has the same application time/temp and the same performance criteria.