However, I've found that once the initial wifi profile is set up through the EyeFi Mobi app, the older app will work with the Mobi card. In the description for the EyeFi (older) app, it says that if you use the Mobi card to use the Mobi app. I shoot in RAW, and when I want to share an image quickly, I'll use the in camera processing to create a JPG, open the EyeFi app and the new image will show up with all of the other images in my image gallery. The older EyeFi app will download any movies or JPGs automatically. The two apps are slightly different as well.
I think the X2 card is more manual setup. It's a plastic card with a code, you enter it in the EyeFi Mobi app and a wifi profile is created.
The difference between the older X2 pro card and the Mobi (and Mobi Pro) cards are how the wifi is set up. I'd consider the Pro models only if I wanted to transfer RAW files through wifi. I think the main difference between the regular Mobi and the X2 Pro or Mobi Pro is that only JPGs are transferred. The two apps in the Google Play Store and Apple App Store are EyeFi (older application) and EyeFi Mobi. I've only used this on the iPhone iOS, so again, your experience might be different. To make things a little more confusing, there are are different apps in the app stores. I've only used the Mobi (non-pro) so I'm basing my judgement on that. The older EyeFi X2 Pro, EyeFi Mobi (non-pro), EyeFi Mobi Pro. I think that there are three major EyeFis still on the market. Every once in a while, I have issues with finding the wifi, but powering the camera on and off a couple of times will usually fix it. As long as the power is on and not turned off by power save, the wifi signal should be created. I use the EyeFi Mobi with my K5 and K-01, even though there isn't an Eye-Fi in the menu. The card itself will create its own wifi, even if your camera isn't "compatible", as long as there's enough power to the card, which usually there is.
My understanding is that you can use any wifi card. I didn't want to deal with multiple methods of handling images so I just upload directly form the memory card now. I also use a K-5IIs which does not support Eyefi directly so that pretty much sealed the deal as far as putting the Eyefi in unused gear drawer goes. It's honestly easier or me to just upload directly from the memory card or via the USB cable.
I also had a 32MB Eyefi Mobi Pro and could not get it to work reliably I ended up sending it back to Amazon. If you're shooting in jpeg or Raw+ and only want the jpeg pictures on your phone then it will work just great but if you want the raw files to go to the computer and jpegs to the phone then it gets a little more complicated but it can be made to work. When it's working it works great but when it decides to take a day off it's an exercise in frustration. The K-50 supports it just fine although sometimes it's a little slow to get started and the setup is a bit finicky.
I have the Eyefi Pro X2 which is the older card and uses the older app on the PC and Android phone/tablet.