ImgIX provides a high degree of flexibility at the front-end of websites. It has a proper caching strategy. With its great power for image manipulation, it fetches the image from the cache and provides a fast and reliable solution. It doesnt add the latency to load the image. It even deducts the latency as its not fetching the master image every time the page loads. Its known to be the best scalable solution for image or content delivery.
Cloudinary free plan is a great offering for CDN users as well. Its free plan gives you 2GB storage, 75,000 images, and 5GB monthly bandwidth. These figures are great for small to medium sized blogs, depending of course on your use of images.
As per your current needs, Cloudinary is a good choice for free CDN services. But, if you have a good setup and expecting a good increase in your traffic in near future, then you always have an option of upgrading to ImgIX. As ImgIX services are quite reliable and fast. Though, Cloudinary seems to be a promising one too.