Identify the strong and weak points of these competitors (for lack of a better term) and hone in on the gaps where your idea can have the most impact.
Your idea can't do everything, but are there any key benefits that competitors have that your idea is not hitting?
Note that you can take items from your earlier User Needs list and enter the most relevant as features/benefits.
It's better to list too many than too few at first, then narrow down and/or consolidate and/or prioritize. Then highlight on the matrix the gap that your idea is hitting.