How about a set of quilted placemats? using the same fabrics and battings in all of them? They are small enough to not use lots of fabric, but large enough to get an idea of how the threads work.
For piecing, I usually use an 80/12 needle.
I've been very happy with the results of (gasp!) the 'older' cotton covered poly Coats & Clark dual duty threads. I may be 'happier' with other threads, but so far, so good with it.
My quilting has been limited to Stitch in the Ditch and/or straight line quilting, and I've used the same C&C cotton covered poly Dual Duty thread for that. I've also usually used the 80/12 needle for that most of the time when working with quilting cotton fabrics.
I go up to a 90/14 or 100/16 when I work with denim.
feline fanatic has a good point - some threads work better for some projects and purposes than others.
It's also important to match up needle size (and style) with the thread and fabric it's to be used on.
If your machine will take plastic or metal bobbins, it may also be a good idea to use only one style.
I think the most important thing in your testing process is to keep the variables to a minimum.