The honest difference
Both services cover the standard PDF toolkit well, and for a single quick merge you will not feel a difference. The models diverge on money and on memory. iLovePDF monetizes through a subscription that unlocks caps, desktop apps and an AI credit allowance. DocFather keeps file tools free and charges credits only for AI operations, so the cost scales with how much AI work you actually do, from zero.
The memory part is the vault: DocFather is built around the idea that the documents you process are often documents you need to keep and query later. iLovePDF deliberately stays a processing toolkit and delegates storage to your cloud drive. Decide which philosophy matches how you work, and if it is subscriptions and desktop software, pick them with a clear conscience.