Big Download notes that regular subscriptions range from $14.99 per month, $41.97 for three months or $77.94 for six months, so the 12-month subscription is substantially cheaper than the $155.88 it would cost for two six-month subscriptions. Remember that the 12-month and lifetime subscriptions are available only for pre-orders, so you'll have to put some money down if you want to save money in the future.
As for whether you'll actually still be playing it in the future, you'll just have to wait and see. Or you could use a tachyon pulse. That works too.