What are pros and cons of Aruba 515 Series AP vs. Cisco Meraki or Extreme Networks?

Depends.  I have personally used both Cisco and Aruba so I am familiar with them.  Extreme I am not, so unless they have some feature that I need or want, I wouldn't consider them.

Do you have experience with any of these brands?  If so, based on your experience would you use that brand again?

Also what other equipment do you have on your network?  Firewall? Router? APs? Security (Cameras, door locks etc.)?  Are there any functions with one of these brands of switches that would enhance the products you already have?  Sometimes you get more bang for your buck by sticking with the same manufacture as they will have features on different products that work together well or feed off of each other.

If all of that comes down to the same three brands still, then look at performance claims, features sets, cost.

We have the Meraki AP's and I like the cloud management portion of them.  But if you don't renew with them to use the cloud then the AP's are worthless as the require the cloud management.  But it is nice to have that central, visual representation and central management feature Meraki has to offer.  I don't know if Extreme Networks or Aruba has that. 

I suggest Aruba. We've considered the same upgrade (APs + Switches) and I've settled with Aruba.

Aruba 505,515 (or other models) depends on the number of users and the area you want to cover. 

Regarding switches, from what I could see, there isn't much difference though my experience is for Cisco so far. I haven't used Aruba switches to judge.

Aruba APs are very flexible: you can operate them in 3 modes: controller-less (Instant) where 1 AP acts as the controller for the others, with the on-prem controller, and cloud controller (similar to Meraki). depending on budget, you can go with a cloud controller (very affordable pricing in our region). then upgrade to an on-prem controller 1-3 years later to split the project budget.

Cisco isn't as flexible: you can either go with the controller-less solution, or on-prem controller if you buy the Cisco Catalyst APs or go with the Meraki cloud-only solution. But you can't migrate the solutions after buying them. As @Michael Fulton ​said: "Merakis are worthless without the subscriptions as you can't manage them".

I have also heard it from several people: Aruba is much better in wireless. Though, honestly, I haven't tried both solutions for myself to give any feedback (Currently, we have 802.11n APs from Cisco).

STAY AWAY from Arista Mojo

Most of the Wi-Fi properties come embedded in the chipset. So, there is not too much difference between different brand APs. 

If you are redoing your Wi-Fi infrastructure because of new technologies (such as Wi-Fi 6), fine. If you are redoing it because of a bad user experience, then you should investigate possibilities of AP misplacement or AP configuration. 

Cisco Meraki is cloud-controlled. If you are thinking about an on-prem Wi-Fi controller, you should concentrate on Aruba or Cisco Catalyst 900 series. 

As far as I know, Aruba 515 has

    - 4x4:4 antenna for 5 GHz 

    - 2x2:2 antenna for 2.4 GHz

Cisco, on the other hand, has either 

    - 2x2:2 antenna for both bands (9105 model) or

    - 4x4:4 antenna for both bands (9115 model)

Of course, whether this difference in antennas will have any impact will depend on the client devices and whether they have matching antennas and whether they also have Wi-Fi 6 technology. 

If you really want a differentiating factor, you should consider Ruckus Wireless. 

Ruckus Wireless APs have;

- Adaptive antenna technology (called BeamFlex). This technology analyses different paths to reach the client and electronically turns itself to a directional antenna, choosing the path that gives the best performance to reach each specific client. If the client is mobile or if the environment changes (such as a warehouse) the selection of paths/direction also changes instantly. 

Transmitting in a directional way allows a good Signal-to-Noise Ratio and also causes less interference for other nearby APs outside the path of the directional transmission.

- ChannelFly technology. This technology chooses the best channel based on performance rather than background scanning for noise. 

- Polarization Diversity. This technology allows the clients not to lose signal strength when held at different angles (important for mobile devices such as tablets and telephones).

- Better receive sensitivity compared to the competitors. 

None of the other brands have the features listed above. 

To summarize: 

-   Features embedded in the chipset, and dumb antennas don't make any difference. 

-   Adaptive antenna and polarization diversity make a big difference.  

I have Extreme and chose them years ago when they were Aerohive for one reason PPSK they are still the only company that has this.  I put out a bid this year and was ready to forklift my entire extreme system because of their outrageous maintenance costs Aruba had a great maintenance cost but to achieve what I needed I would have to purchase clearpass an excellent product I might add.   TCO for Aruba was about 10% of my original Extreme quote.  The bid for extreme came in and they had dropped the maintenance cost to 10% of the original cost for 5 years.  I would like to have purchased Aruba but I have 125 Extreme WAPs deployed so the cost of Aruba forklift would have been higher and the need to retrain techs on how to configure Aruba is now not an issue.  If I was doing a system from scratch I would choose Aruba hands down for their service, price and ease of use.  I have a full Aruba network of routers and switches as well as all HP printers so it would have been a good fit. I must add this is based on educational prices I am the network Engineer for a residential High School ASMSA.  Your prices may vary.  I have heard from other extreme users that their support is not what it was when Aerohive was a stand alone company, I have a very knowledgeable vendor so I rarely have to contact Extreme.