Right, some facts:
1). The blitter is the same speed in the A12oo as the A5oo
2). 16.8 million colours, max 256,000 on screen through HAM-8, which is pointless as it takes up too much DMA time to be anything more useful than static pictures
3). Sound channels are clearer, and if you remove one of the sound leads from its socket, all channels will be played through the other connected socket.
4). 14Mhz processor with caches which means processor run games are quicker.
5). Greater display modes
6). 2 Meg of chip ram which is useful obviously.
7). Bigger sprites (64 pixels wide) and Overscan bug is fixed where you would lose sprite 7
whilst one of you is correct in saying ECS 32 colours AGA 256, the fact is most A5oo games used a lot more than that via the copper. With the exceptions of muppets like Tiertex, Kremlin, Teque, most games were inexcess of 200 colours, simply do a screen grab with WinUAE and then get your favourite paint package to count the colours.
Oh yeah, you could also quarter pixel scroll on the A12oo whereas the A5oo would scroll pixel size, and the copper could reload the screen quicker on the A12oo (4 pixels as opposed to 8pixels on the A5oo)