Honestly, you will only notice the difference between a 2x2 and a 3x3 mimo environment when most of your clients support 3x3. Co-channel interference and contention will limit the throughput on most networks unless you can match your clients to the capabilities of your access points. You should deploy 3x3 access points because that is the best available and if you do not control your population, there will be more and more of those clients and you want to be prepared for them. You should also deploy 3x3 if you know you will not be replacing your access points for years and you want to be sure that it will gracefully be able to support the latest and the greatest.
At this time 802.11ac is not even a standard and it will take awhile for clients to come out that support that standard, much less REAL backhaul on most networks that can support over 1 gig of throughput.
Again, most mobile devices have applications running on them that have been optimized to be in such an environment that has alot of contention and loss and they are designed to use lower bandwidth and deal with disconnect fairly well. The most expensive solution is not always the best, but at minimum, a dual band solution will offer you the best bang for the buck.