Is it better to do a BEng (Hons) and a one year MSc or an MEng?

The 4 year MEng is the one to do. And if you get a 1st or 2.1 the better (easier to get CEng status latter on)

If you are a UK student it will be easier to fund a 4 year MEng, than a 3 year BSc plus an MEng.