The 996R is an iconic bike that will retain far more of its value.
The 998 will NEVER be in the same league ....
And I doubt either you or I are good enough to either notice or utilise the performance differences ....
Personally, my money would be on the 6 ........ but I haven't got any money so my opinions rather irrelevant .... lol
Get a 999R, they are getting cheaper by the day if you want the last ounce of performance .....
Whatever you do, buy what YOU want ...