Consider going with Mikrotik gear instead. It's the same hardware, but you aren't locked into Ubiquiti's proprietary Java cloud management software - everything can be managed with SSH if you prefer. It isn't as user-friendly or as pretty as Ubiquiti's web interface, but you can get deep down into all kinds of internal settings and it also ends up being significantly cheaper.

For example, three dual band triple chain 802.11n/ac APs (https://routerboard.com/RBwAPG-5HacT2HnD) and a router (https://routerboard.com/RB750Gr3) have an MRSP of $330, including POE for the APs.

I have a Routerboard Hex and it's solid. It was a fiddle but I managed to get port isolation working so I could add a guest WLAN. This was more for 'fun' than any real need :-) A foundation in IP and routing is definitely beneficial for these devices. Due to its excellence I have been eyeing their Wifi solutions.

Any good setup guides? One of the things I was hoping for was a good central management system, plus good roaming and handoff. I currently have two DD-WRT flashed ASUS routers, and having a hell of a time with handoff when going from one floor the the next.

They have recently introduced centralized AP management called "Capsman" but it's still a little rough around the edges to set up. Their wiki has a lot of good info though, eg http://wiki.mikrotik.com/wiki/Manual:CAPsMAN

Roaming is very tough to get right unfortunately. Only the client devices can make a decision to roam, so without ugly hacks such as using a single channel / MAC / SSID to create a giant interfering WLAN, your options are limited. What you can do with Mikrotik is set up access rules to kick a client off an AP if the signal strength falls below a threshold, but as this is not client-initiated, there is a longer re-association time which could interrupt VoIP etc.

So how do you handle roaming in your situation? Do you use different SSIDs between APs? Currently I have the same SSID shared between the two Asus routers (one configured as AP only).

Right now my apartment is small enough that I only need a single AP, I have it wall mounted high up in a central location so it has good coverage of the whole building.

There are options such as 3rd party roaming clients for (eg https://play.google.com/store/apps/details?id=com.heleron.wi... for Android) which I'd consider. For laptops, most drivers have a "roaming aggressiveness" setting which can be tweaked. In general almost all devices should handle transient connectivity glitches fairly well, so kicking low signal clients off an AP shouldn't interrupt too much.