More is always better when it comes to bandwidth related features. However with access point spatial streams there are also practical considerations with respect to the cost versus benefit of a 4x4 AP over a 2x2 AP and the right solution for a deployment could be more nuanced than just biggest/fastest.
Client distribution is still leaning towards being 1x1 or 2x2. The majority of phones and tablets and most laptops are still 2x2 or in some cases just 1x1. In some deployments the number of portable client devices that have more than 2 spatial streams is rare enough that network administrators may not feel the cost premium of 4x4 is worth the benefit. This is especially true of rural Wi-Fi projects, or some of the free Wi-Fi deployments.
Newer clients that support MU-MIMO can leverage multiple transmits in parallel from an AP, which makes 4x4 an attractive option for high density deployments with a majority of new clients. Practically for MU- MIMO to work and show its benefits:
- the clients have to support it,
- there has to be data to be transmitted to multiple clients at the same time,
- the clients have to have enough spatial separation that a simultaneous transmission to both can be achieved.
All of these conditions have to be met together, and this is usually the case only in more urban high density deployments. Also, most 4x4 Access Points need a power draw more than 802.3af provides for their full performance, while 2x2 APs can typically manage within that. So going from 2x2 to 4x4 also brings up cost of upgrading the PoE switch infrastructure.
In the middle of this lies 3x3. The market saw the advent of 3x3 Access Points back in 802.11n days. However with 802.11ac there are enough 4x4 solutions at similar price points that anyone considering a 3x3 solution could just switch over to a 4x4 (from a different vendor if necessary) and get the additional MU-MIMO groups, being able to service more clients and get better efficiency.
P.S: and for anyone wondering about the horses for courses note in the title, its an idom of British origin: https://en.wiktionary.org/wiki/horses_for_courses