Hi,
You mentioned that games are not a priority, so why would you like to have AGA? A graphics card will give you much nicer Workbenches, and you've already got an SD/FF.
The 4000 has the option of fitting a Toaster system. They sometimes sell for very low prices these days. I think there are many more options for graphics cards as well (though some of those options might be rare as hen's teeth, and not really all that great). If you want to do real work on the machine, the HD throughput can be a big factor too.
I don't know what the situation is like in the UK, but in Aus, 1200's are much more common than 4000's, and dirt cheap by comparison. Accelerators are more plentiful too.
I'm not sure what the situation is with PPC's on older these machines, but I'm under the impression that they are not actually used for bery much? Can anyone correct me on this? I think most software only takes advantage of whatever 68k is available. I've never used one though, so check this point. However, I think the difference between using a PPC on a 1200, and a PPC on a 4000 will not significantly affect the overall experience.
edit- Occaisionally I've liked to port my 1200 around, to use on the road, or just to plug it in to a friend's TV etc. This is one good point of a wedge. Some people still use them for gigging. I've never done that though. In the good old days, the 1200 was considered a reliable gigging sequencer, as PC's were famous for failing once they filled up with smoke in a club.