Since it was started with Mettler thread, the block requires precise quilting for matching, and you have Mettler thread on hand, I would finish it up with Mettler to keep everything consistent and use up the thread. I know what you mean about the lint, though. You could certainly switch to Aurifil as long as you take care to check that everything is lining up as you should. If, for some reason, it isn't (could be the change in sewing machines as well as thread) you can just make a slight adjustment on your seam size when sewing the blocks together.