Some of it is basic practicalities. What kind of quilting can you do - is it by hand or machine, are you comfortable quilting curves or are you happier with straight lines, are you able to do fancy advanced stuff or do you prefer easier motifs? Then there is the issue of how much time you want to spend on the quilting. Another point is the batting: there is a maximum distance for quilting lines which varies depending on what sort of batting it is (cotton can be quilting further apart than polyester, for instance), and if you quilt further apart than the recommended maximum, then you may get problems such as the batting shifting. 4" is a common maximum distance for poly, I think it's about 8" for cotton, but it may vary by manufacturer.
I did stitch in the ditch for my first quilt too. I was intimidated by working out a quilting pattern as well as a piecing pattern, and to be honest I had used the kind of piecing pattern that was quite busy enough already (Carpenter's Square). My quilting has gradually become more adventurous over time. At the moment I'm very inspried by Welsh quilting, which you can see examples of if you put "Welsh quilting" into Google Image.
I strongly recommend Google Image for ideas in general. If you're doing a certain quilt block, let's say a Jacob's Ladder, then you put "Jacob's Ladder quilt" into Google Image, and it will bring up thousands of examples of how other people have made that quilt. There are usually plenty of images which show enough detail that you can see what sort of quilting patterns were done.
I like to keep a folder on my computer of inspiring quilting patterns, and when I'm stuck I look through them.