The cool thing about the a1200 is its ability to 'grow' with your demands. At first, its just as with the A500: a cheap alternative to the big box machines that can play all those cool new games. And believe me, there are loads of AGA games that you're not able to play on your old ECS machines. Then, after some time, you run some productivity stuff and find out you want a harddrive. After that, you get an accelerator, a tower conversion, graphics card etc etc. Each time it makes the machine even better than before, each time it is able to meet your demands.
Last year I had to decide between a stock a1200 and a stock a4000/040 and in the end I went for the a1200 route, selling the a4000. Its just much more fullfilling to have an entry-level machine expanded to the max, especially when you can keep it all in its original case.
Back in the day, I did all that A1200 upgrading, and eventually realised what I was trying to have was an A4000 with a graphics card. And TBH I still think a souped up and reliable A4000 is the way to go for expansion, with the exception of not having PCMCIA networking if thats important to you.
If you want to do chipset graphics and sound, play games and have a cool-looking Amiga which occupies little desktop space, then get an A1200. But IMO RTG and CDROM is a lot less hassle and neater in an A4000.
