LES vs. NETS

One of the primary differences between the two has to do with how names of people and places are rendered. The LES does its best to make names familiar to those with history of only reading the Bible in English. NETS pretty much transliterates person, place, and people-group names. If you don't know the Greek and aren't familiar with these names, it can get pretty difficult to track who is doing what in some passages.

NETS uses Göttingen (where available) and Rahlfs otherwise as its source text. LES uses Swete's edition of the LXX as its basis.

NETS implements a translation technique they call "interlinear" (not what you think, you should read their front matter to understand it better). They also make a point of trying to use NRSV vocabulary (well, except for names). LES translates the Greek as Greek.

The LES actually began as an interlinear (Lexham Greek-English Interlinear of the Septuagint) which was then edited (some places more heavily than others) and smoothed into the LES.

Those are the larger differences in philosophy, method, and implementation. Hope it helps.

I don't think it is an either-or proposition; I think having multiple English translations of the LXX is a good thing.