How to compare performances of two databases
You can not compare the performance of 2 databases by just looking at their sql plans. The same queries can have very different plans. When the versions are different all kinds of computations for the cost are different. The plan that a query follows in execution could very well be different than the plan you get from explain plan from your client.

The only way to compare the performance is to measure the execution of the queries or processes and compare those timings. You could use AWR to compare the database performance. A certain workload will generate a certain thoughput and consume more or less dbtime. If the dbtime spent and the elapsed time of your jobs is lower, you can safely say that the performance in that database is better compared to the one where the times are higher.

In 11g, with the auto tuning tasks enabled and auto accept sql profiles enabled, it could very well be that the next run already shows better timings.