I don't have the Addis so I can't compare. The Signature needles are not true interchangeables. The call them convertibles. Each size tip has its own set of cable. I love the Signatures inthe smaller sizzes, but find them heavey in the larger sizes (above 7.)
I have Signature in all sizes of circulars, convertable and fixed. They are wonderful! I do not have Addi's. On the KP forum people say that the Signature points are much better for lace than the Addi lace needles. Signature does not make sizes below 3 in the circular needles so I order Dynk interchangeable in the small sizes. They are great as well but the cables on the Signatures are much better. Hope this helps!
I have Signature Circs and Addi Lace - both fixed circs. I am not a fan of the slower finish on the Addi Lace fixed circs, but find that it does wear off over time. The tips are sharp enough for lace. And I knit a LOT of lace.
Because knitting tools are all about preference - and my preference is needles that are slicker than snot - I most often knit with my Sig Circs or if not something in lace, my Addi Turbos with their original fast finish.
My needles are the tools of my craft. They enable me to do what I do well, so their cost is secondary. And I have had my "muggle" family gift them to me for holidays and birthdays so I would have what I really wanted and could use.