The watermark difference
For most people the single biggest factor is the watermark. Remini's free results carry a watermark, which makes them unusable for professional or commercial purposes unless you subscribe. ImageUpscaler does not watermark your output, so what you download is ready to use.
If you only need occasional mobile selfie touch-ups, Remini's app is convenient. If you need clean, watermark-free images for a website, listing, resume, or client work, a no-watermark tool removes that barrier.
Web vs app
Remini is app-first, which is great on a phone but less convenient when you're working on a laptop with files in folders. ImageUpscaler runs entirely in the browser — no install, no app store, and it works the same on desktop and mobile.
One tool vs a toolkit
Remini focuses on portrait enhancement and restoration. ImageUpscaler bundles several related tools — an image upscaler (up to 10X), a photo enhancer, an AI headshot generator, profile-picture tools, and a color palette generator — so you can handle a full image workflow in one place.