make sure the smaller seattube frame doesnt also have a shorter top tube , it could be made to fit with a longer stem but you start changing the way your used to having a bike feel so its worth finding out all the lengths and angles before you lay down full suss money
Being that it is an LTS, 16" is perfect for the gent who normally would throw the leg over an 18" hardtail. Top tube measures up about the same (along the horizontal) of most of my 17.5" or 18" HT bikes. Buy with confidence, my good man!
I'd also say you'll be fine with a 16", i can just get away with an 18" steel frame but an 18" LTS would be too big (my 16" is perfect), also alloy GT's are measured centre to centre so are a big 18" (compared to a frame measured centre to top) and generally the bottom bracket height is higher on a full susser making it feel bigger still.
when I got my first RTS years ago I was told to get a smaller size for a full sus .
so I ride 18 for hardtails and when i have full sus , i get a smaller one .