Give it another month for mini-itx H55/H57 motherboards, it's a lot of brand new traces to have to sort out.
The difference between P55 and H55/H5 is that P55 is that the H55/H57 boards are designed to work with the chip integrated graphics, memory controller and I/O hub on the Core i3 and Core i5-6XX series of processors, the P55 chipset has it's own memory controller and I/O hub, so it just disables the chip integrated ones where applicable.
The best feature/performance board IMHO is the ASRock board, but if you want SATAIII and USB3.0 then you're best off with the ASUS H57 board, I can't justify the extra price personally.
For a HD5450 in Mini-ITX you're pretty much limited to the Zotac socket 775 or ION boards as there are not many mini-ITX boards out with PCIe X16 slots unless you want to go all the way up to the $200 DFI P55 mini-ITX board.