i would encourage you to try the adizero mana...i just got a pair for a fall marathon myself and am really pleased so far.
1) you are already wearing adidas and they apparently fit you well
2) they are blue and that is really cool
3) the manas are only a fraction of an ounce heavier than the RCs
4) the T6s are a fairly "odd" shoe
you really need to try the T6s on to know what i'm talking about. they might not feel that odd to your feet, but in my opinion the cushioning that is there is quite firm, the shoe is pretty darn narrow (especially in the forefoot and toe area), the heel is higher relative to the forefoot than most competitive shoes, and the forefoot sole is much less stable than the adidas mana. whereas the mana forefoot is really flat from left to right (like the mizuno wave musha if you've ever worn those), which inherently provides stability during footstrike or pushoff, the forefoot of the T6 is narrow and sprung, both of which tend to encourage the foot to collapse inward late in the footstrike. i've considered both the T6 and green silence (which is much softer than the T6) for a marathon, but both shoes encourage this very late stage pronation which really scares me out of wearing them.