Worth is something only YOU can determine.
Yes, the 7950 is faster than a 7870, but it is only one level higher on Tom's heirarchy chard. That is not so much difference that you would notice it outside of a synthetic benchmark.
If you have the funds go ahead and buy the 7950, otherwise you will always wonder if you should have.
It is the same rationale for those picking a GTX680 vs. a GTX670.