The first case behaves like To<MyService>() except that you explicitly select the constructor. This means the context is passed through MyService and you can use conditions for IFoo and IBar or one of their dpependencies where in the second case you get a new context for IFoo and IBar and you will not know that they are injected into MyService.
e.g.
Bind<IFoo>().To<FooA>().WhenInjectedInto<MyService>();
Bind<IFoo>().To<FooB>().WhenInjectedInto<MyOtherService>();
will not work in the second case.