The recent Barenreiter complete edition is by far the most comprehensive and authoritative, and is also beautifully produced.
There was an ABRSM edition of the SWW on sale at a local shop that I grabbed up. At the time it seemed like a good purchase, but upon closer examination at home not so much. The editors had made decisions to replace some of Mendelssohn indications with their own. What is even more worrying is that there is no differentiation in markings between what is Mendelssohn and what is editorial. IMO, avoid ABRSM edition for the SWW. There is nothing wrong with an edited edition. The problems begin when editorial elements are not clearly disclosed, or the reader is put under the impression they are reading the composers indications when in fact they are not.
Sorry, I should have added 'of the Songs without Words editions'.
Just searched the price for the Barenreiter: $47!