Windows Phone performs pretty well even on the 520, but just keep in mind that it's not in the same class as the mid to high end Lumias. I was using a Lumia 800 previously and while I like the 520, I do miss some things. For example, no LTE data speeds, weak camera with no "flash", and the screen is very reflective and mediocre compared to the amazing screen that my Lumia 800 had. So yes, I would focus on the OS and apps.
The services you choose to use will play a big part in how well you can tie your devices together. You may someday decide that you WANT to replace your iPad with a Surface or other Windows device so that you get a more seamless and capable experience, but it isn't required. MS has been very good in recent years about making their services and software cross platform. Actually, it's Apple that attempts to lock their customers into their ecosystem with everything from proprietary charge cables and music/video formats to devices and standards that only work with their devices. Google in general is pretty open, but there have been several cases recently where Google has disabled/broken their services for competing platforms (Windows Phone has been the target more than once).
Some examples...
MS Skype vs. FaceTime/iMessage.
MS Xbox Smartglass or Windows 8.1 Miracast support vs. Apple TV or Google ChromeCast.
MS OneDrive vs. iCloud or Google Drive.
MS Office vs. iWork or Google Docs.
There are plenty more. Each of those MS services/products is either an open standard or available on nearly all competing platforms. On the other hand, the non-MS products have little or no availability on competing platforms.
The 1520 is the newest flagship level phone and it is a very nice phone if you are into phablets. I'm not, so it's not really my thing. The Lumia 1020 and 925 are the "best normal size" phones on AT&T in my opinion. The Lumia 930 (rumored to be the same as the Verizon Lumia Icon) is supposed to be coming out very soon.
This is Windows Phone's weakest link. Most of the common apps are there, but some of the more specialized ones have not made it to Windows Phone yet. Personally, I'm not that big into apps and there is nothing left that I need that is missing from Windows Phone, but that isn't the case for everyone. Only you can decide what you need in a smart phone. Here are some things to consider though...
- In the beginning of the modern smartphone era, many apps were created on iPhone simply as a feature limited substitute for a full featured website because the iPhone browser experience was so poor and it couldn't render complex websites properly. Later, people just came to expect an app for everything and they seemed to have forgotten that this thing called the World Wide Web still exists. Windows Phone IE can render webpages as accurately as the desktop equivalent with the exception of websites that use Flash. In these cases, an app is not really needed on Windows Phone. Of course there are plenty of apps that do things that can't be done in a web app, but it's something to consider. For example, I have a somewhat regular need to check the marine weather forecast or tide info. I know some friends that "have an app for that" and Windows Phone has some too, but I don't bother with them because the webpage is bookmarked in my IE favorites and it does exactly what I need more quickly than an app.
- Need vs. want. I'm not suggesting you limit the usefulness of your smart phone experience, but I do see a lot of people installing apps that are just silly time wasters and some of those people look down on Windows Phone because those same useless apps aren't available. I have seen people get talked out of trying Windows Phone the moment they find out one app isn't available even though they have no actual need for that particular app. For example, several months ago, before Instagram was available on Windows Phone, a lady I know got talked out of buying a Windows Phone because the sales guy told her Windows Phone doesn't have Instagram. She is 60+ years old, doesn't even know what Instagram is (still doesn't to this day), and she really wanted a Windows Phone when she entered the store. Anyway, my advice is to look at the big picture and don't base your entire decision on one app unless you REALLY NEED that app.