It debends what you want your project to do, MDI stands for Multiple Document Interface, and it does exactly what it says on the tin, it provides an interface for multiple documents, If you want something like Word, where you can have lots of the same type of form open at once, all inside one big form and you work on the one that's highlighted, then use MDI, if you want lots of different windows open at once use normal forms, MDI is great for word processors and the like but not much good for other things.