HP dropped support for my wireless gear and I need to look at replacement options.  Any recommendations?  I currently have an MSM720 controller with 20 HP AP's of various types.

That is a complicated answer. I currently have deployed 2 Cisco 5508 WLC in an SSO HA pair. With 336 AP deployed. It works very well for me. The baby controller standalone 2504 works the same with just less AP and client support. 

Tell us about your environment. Is this a single office with all office space. any warehouse or outdoor coverage needed? Any features that are nice to have or would be nice? Or do you want something that just works and you want to set it and forget it?

Goofy horseshoe shaped 2-story office complex... current AP layout provides full coverage so I'd be replacing the AP's 1 for 1.  Current HP system is configured as a client isolated guest network.  Employees must use VPN to access the wired LAN.

Normal load on the system is about 70 wireless clients.  That could go up to 200 or so if we're hosting a large public event.

I would like to give employees the ability to authenticate their domain credentials over WiFi and access the LAN without VPN.  We frequently host students and short term visitors, so I need to maintain an open guest network.  We are a non-profit so I would like to avoid overpriced AP licenses and planned obsolescence.

First ring out if your non profit is ellegiable for Tech Soup if they are that's good news. 

I would give give a serious look at Meraki. Easy to setup and use. Pay attention to licensing with this. However it's one of the best wireless products on the market.

Traditional Cisco is another great option. Look at the 1850 mobility express. It's easy to setup but it's newer on the product line runnng early code. Mobility express has a limit of 25 APs.

I do have access to Tech Soup, but don't qualify for Cisco donations (primary mission must be serving the poor).  Retail on the Cisco AP's is pretty steep... $5-600 range.

Any experience with Unifi equipment?

HP bought Aruba which makes some hot shit wireless gear.  If you can tolerate HP and get a deal its good equipment I have run in large enterprise doing reliable solid and secure work.  

HP/ Aruba licensing was always an icepick in the balls process.

I have never used Ubiquity on that large of a deployment. Just make sure you setup the softwar controller. It can be hosted in the cloud for around $10/mo. You will need a firewall and separate vlan for secure guest wireless. I'm assuming you have that already.

Also look st something called packetfense for your NAC. I have been using it and like it a lot. If you have users that need to be on the guest wireless but have valid credentials it's a good option and it can help keep thing secure. Also it allows for email and SMS based guest registration. 

Despite what Ubiquity tries to market, they are not an enterprise-class solution.  Cisco, Meraki, and Aruba are the big players.

Meraki is probably your best option 20 ap assuming standard discount is $11,400 will take you longer to install them than configure it. MR33 AC wave 2 and a 3 year  license. I don't know if they still have the free 1 year promo with that. 

Look at fortinet too.   I didn't like unity had lots of issues with the waps

Meraki can be had for a fair discount - I have a quote for an MR33 at @403, with 3 years of service at $186, 5 at $279. (need AP + service)

I've setup Unifi gear, but only in a small (2 AP) environment. The controller software is pretty nice, the APs aren't stupidly expensive, and they'll keep running without a monthly 'license' fee. You have to be careful though, as not all of their gear uses normal POE. I can't tell you anything about how they work in triple-digit installs, but I can say that they will do RF surveys to automate channel selection.   I see them as a Meraki Light. Just about a ProSumer price point, about corporate capability.

I will suggest a dedicated management box for them though. They all get tied into it, and its been a bitch when that one specific box is not available to make a change. (cloud key dongle, dedicated VM, anything but a daily use system that may get upgraded/reimaged/replaced at will)

They seem to be getting better over time, but I have to agree in general. I'm looking for a full-stack to replace my current gear, and I'm just not comfortable putting in their security boxes, switches and APs, all in one big lump. I am pretty comfortable with their APs, but I'm not going to bet my job / reputation on the others. 

Yeah... I'm thinking in the perspective of an SMB. I'm withdrawing my so-so endorsement for your environment.

We were running Aruba APs and when doing our network upgrade last year we looked hard at Meraki. We really liked them but there were 2 issues. First they don't support Linux for mdm and will not in the foreseeable future, and we have a lot of Linux clients, and when your support runs out the devices stop working so you have to always keep support but when they are at end of support they will just be a brick. We also liked that we could manage remote site networks from the cloud and have more of a global single pane. When we were going to purchase them, we were going to purchase with support until the end of their support life. The lack of Linux support for mdm killed the whole deal though. 

We purchased Aruba again and I just copied the config from the old APs to the new APs and then deployed them. Pretty simple for an upgrade.

You can do one of Meraki's online webinars and they will send you a free AP or switch. They sent me a MR18 with 3 years of support after I did a webinar with them. We got to play around with it and test it which was nice. As it was not of much use to use to us at work, I took it home and use it there until the license ends in another 2 years.