As to why 2 Gb card in the HP2, only Uniden probably knows, but you can put larger size (memory) cards in the HP2, 436, and 536. Up to 32 Gb I think.
I have all of these scanners, and they are all pretty good. However, if you are only going to get one of them, then there are other considerations. I always tend to favor the 436HP or 536HP, but that is with the caveat (condition) that eventually the analyze function and WiFi option are available. The 436HP is a true handheld (albeit it large) and the 536 is a true mobile/base. The HP1 and HP2 are kind of clunky for portable use. But if you are going to use them at home most of the time, or mount in your car, or take to a hotel with you, they are great.
Hands down, the color display on the HP2 is clearly a winner in terms of visual appeal. And the extreme analyze features are an obvious advantage. Frankly, even if the analyze features finally become available for the x36HPs, it is going probably going to be less visually stellar than on the HP2.
I like the QKs on the x36HPs, and if you like to tinker with a lot of settings (as I do), the x36HPs offer more settings. The x36HPs also have a true NFM filter, whereas the HP2 does not (this issue has been discussed extensively on other threads).
That said, if I could only buy one scanner that included the ability for P25 phase II at this point, I probably would go with the HP2. I think for the majority of people who want to have a good listening experience and a visually appealing interface, the HP2 is simpler to use, and very good for what most people need in this modern day of digital trunking systems. For the hard core scanners (I include myself) that like to play with a lot of different features, every scanner available has its pros and cons. Each of the scanners I list below have certain features that I like.There is no one scanner out there that will meet everyone's needs.
Steve AA6IO