A side-by-side NAS comparison tool (pick two or more models, see specs and pricing lined up) is in progress and not live yet.
Every Synology and QNAP NAS model's full spec sheet, support status, and current pricing is already searchable in the NAS model database -- open a couple of model pages in separate tabs for a manual side-by-side.
See also: storage planning for camera recording and the rest of the buying & setup guides for help narrowing down a model before comparing specs directly.