which thread to use??

[Quiltfreestyle]

If it was me I would buy the big spool. Those are usually 1200 yards or more so it will be more than enough to do your quilt.
There is an app for iPhones from Superior Threads that helps you calculate the amount of thread you need. It's not perfect but it will give you a ballpark amount. I got it from the App Store & it's free
I've used it several times & found it helpful, so far I haven't run out of thread when quilting.
I personally use either Superior Thread or Connecting Threads threads but keep in mind I have a long arm machine that I use to quilt on.