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"Best" option would likely depend on other features that might be necessary or useful. The 6500 offers much higher performance than the 7200, while the 7200 offers more features.
PS:
If you go with the 6500, you might also consider using just one chassis, with (e.g. 6504) or without (6503) a 2nd sup. 7600 series might also be considered as its feature set is more routing oriented.