I think its easy choice, at least for me.
A77 when i need fast AF speed, weather sealed and fast bursting shots. Eg, zoo, events or some outdoor outings.
Nex5n when i was to be discrete, travel light and shoot good low light photos.
point to note is that from all the review samples i have seen, its true 5N has better low light performance, from iso3200 onwards, its probably 0.5-1 stop better than A77. However, with more A-mount lens choice and the new f2.8 kit lens, A77 can anytime shoot at least 0.5stop at wide angle and 2stops faster at tele-end using both kit lens, so in a way negate the advantage that nex5n has, unless you hoot the cz24 lens when its out but its not cheap.
When outdoor or anything iso1600 and below, theres no difference in IQ for both A77 and 5N, but A77 has more native A-mount lens for better IQ.