How important is AP support for 4x4:4 MIMO vs. 3x3:3?

Comparing Aruba's AP-305 to AP-325, the major difference I'm not sure about is the MIMO chains.

Even though the vast majority of clients support only 2x2, there are (marginal) aggregate performance gains from more chains on the AP thanks to downlink MU-MIMO, beamforming, and lower rx error rates due to MRC.

But is the fourth tx/rx chain in the 325 really worth twice the price of the 305? My feeling is I'd rather have double the APs for better density/capacity with 3x3 APs, vs. half the APs with 4x4. Interested in input.

Edit This is an honest question without an obvious answer that I can see. Please explain instead of just downvoting, thanks.