MSci vs MEng for Computer Science

Pretty much just this. Wondering whether there is any legitimate difference at all in what each degree will give me in terms of maybe applying for certain jobs post graduation. From what I can tell, it seems pretty irrelevant to the actual course on the most part, particularly when places like Manchester offer an MEng for 1 additional year to their BSci course. Any advice on what to consider? Or is it largely irrelevant

Having an MEng won't give you any advantage over someone with an MSci (and vice versa). It's just the style of degree that universities want to apply.

Both are undergraduate/integrated Masters degrees and require you to do an extra academic year on top compared to a normal BSc/BEng

Okay that's good then. However, presuming there is no difference in desirability and just out of curiosity, what differences are there, or why is there this variation between Universities and especially between 3 and 4 year courses?