6 months is safer.
Pay for 12 months = Get 14 months usage.
Pay for 6 months x 2 times a year = Get 14 months usage.
So it comes up to the same amount after 14 months.
No reason to go for the 12 month plan unless you want to forget about paying it for 1 year.
Currently,
ACT Blaze (999) users pay ~Rs. 1139 for 40GB at 20 Mbps.
ACT Storm(1099) users pay ~Rs. 1253 for 60GB at 20 Mbps.
If ACT Storm users move to ACT Blaze for the offer, it will come up to ~Rs.976 per month for 65GB at 20 Mbps.
Might be worth looking into.