quiltsRfun has excellent advice. If you haven't tried to free motion quilt and have a hopping/darning foot, try that on a practice quilt--you may find that it's easier once you get the hang of it than straight line quilting (fewer puckers).
The 30# thread will definitely be more visible than the 40# thread. If you do decide to use it, make sure you have it set up right on the machine (I use a wide coffee cup to put it in behind & under the first thread tension guide. Also, the thread contrast with your backing will matter on how visible the quilting is on the back.