Im agree with Ranking
You actually cannot compare both devices, ArubaController just does a way more than the Cisco controller
Remenber that the Aruba controller got build in Firewall, build in IPS/IDS and so on.
Cisco WLAN controller dont have anything of that... they sell that in another box as far i know...
This is the reason of why the Aruba controller does support less APs.
Have you told your client everything you can do with aruba controller? what you can do with Firewall licnese? IPS/IDS ?
I want to see Cisco quoting all t he boxes toguether to see how much the budget goes to the sky, just to try doing what the aruba controller can do alone.
Bealive me you just need to see what are your clients needs, or create those needs to them. We have won agaings cisco many times to clients that just got cisco everywhere... now they got cisco Everywhere but Aruba on wifi. :)
It would be nice if some of the users here with big networks with more than 4k aps can tell you their expirience with aruba it could help though... like Ryan, i think the university he works for, got like 6k APs