ofab is easier to upgrade because the map sensor can be changed easily to run more other than that there isn't much difference.
i was advised to go with ofab when i changed mine and have had no problem with it at all- i have heard of some teething problems with ofac on the other hand.
If your gonna run a 2.1 zvh though i agree with karlos you need a live mappable system or you'll never get the best out of it