You match the needle to the thread you're using, or vice versa, as well as what you're trying to sew. How many layers and how heavy is the canvas? It may be that you're wanting the machines to do more than they're capable of. If it's just 5 or 6 layers of light canvas, an 18 jeans needle should do it. I don't think either of those machines can use a bigger needle than that.
Cari