Hi Deepali,

I have found a collection of information for you which may help you in the short term.  There are new videos and comparisons being made with the next release of the products, as DOORS and DOORS NG close the gap items.

For more in-depth answers on each of your questions, please see the sections here:
https://jazz.net/products/rational-doors-next-generation/

In addition, this forum post should be useful Licensing: See https://jazz.net/forum/questions/98343/installing-doors-ng-401-with-doors-9-licences, but also note that you can use tokens

Stability: DOORS 9 is a very mature product, so you would expect it to be slightly more stable.  In terms of new features though, please see https://jazz.net/downloads/rational-doors-next-generation/milestones/6.0M10?p=news which would present a paradigm shift in RM with DOORS NG

Scalability/Performance: Depending on whether you want very large modules all within a LAN, or you want to collaborate across the WAN with thousands of worldwide users.  They both scale very well, but each product has its own way of achieving your requirements. 

There is also an article written to compare the two tools, which probably answers more than the 6 questions in this post and in some detail
https://jazz.net/library/content/articles/doors/4.0.1/which-rm-tool/which-rm-tool.pdf

Finally, there is a useful video on the differences in the toolset from a year/18 months ago.  This is the worst case scenario and as I said, there is a more up-to-date version being authored right now
https://www.youtube.com/watch?v=7ja0T8OwDk4&list=PLZGO0qYNSD4V5mE7UZJhuRmRZtmjrjwwK

Kind regards,
Paul