Hi Vaman,
HR Renewal is nothing you can compare with Successfactors, because it is just a bit mire than a new UI for (parts of) SAP HCM, whilst SF is a complete HCM solution.
So, what you need to compare is SAP HCM (incl HR Renewal) with SF.
By far the biggest difference is: SAP HCM is an on-premise product, whilst SF is a cloud solution - mostly SaaS with an element of PaaS.
This is a fundamental difference. A bit like comparing a car you own with a full network rail pass (think of a very old car, which has has a lot of uogrades/tuning to keep it modern, but needs taking to the garage often with a railpass for a very modern rail system like the one in Japan.). The railpass will get you to most places quicky and comfortably, but you there are a lot of small villages you can't get to at all.
You MUST understand the technical, cultural and economical implications of this difference, if you think about switching.
Only then does it make sense to look at details of individual features.
You can easily fill a book with this comparison, so you beed to look at it in your bespoke context and then ask more specific questions to the community.
Kind regards
Sven