Bad logic in this thread.
1) The GTX 560 ti is a little less expensive but also a little less powerful.
2) Factor in the usual driver arguments regarding amd's drivers having a reputation for being less good (doubly so for crossfire/sli)
3) "I play at low res, so both are overkill, so card a is better" isn't a good argument unless you're saying card a costs less and is less wasted performance than card b. If that is the case then another, cheaper card that still maxes performance at your low res would be even better (perhaps a 560 non-ti or a 6870).