Paul,
Having dealt with SLS quite a lot, I know that MB contract out the manufacture of many 'MB original parts' to SLS, then buy them from SLS, add their mark up and sell them on. One reason why many SLS parts are 'original' MB parts. Also, as has been pointed out here on several occasions, MB term any part to be 'original' even though it may not be the truly original part that was fitted at construction, i.e things/design/methods of construction all change over time.
My point is that you may well find there is no difference between the 2 tanks - other than the price, of course. You could always ask SLS; I am sure they'll tell you. They've saved me a few £ that way already.
James