Not really too confident in my knowledge of the heads, but I'll give you what I got. There are really three heads for the M10, the difference is really in valve size and the domeage which is a variable for the compression. The real problem with the heads is the camshaft drive system, not sure if the flow is the real problem here because a good camshaft can solve some problems. The problem with the cam drive is that it cant use roller rocker setup, some people have tried but the way it works with the distibutor is the limiting agent and wont work. Many people try to get the most from the cam drive with a better cam and lighter parts like titanium valves and rockers. Most people go with the M20 route because there is more of an aftermarket for it and parts can be cheaper since the M20 is the engine from the E30 which is more "mainstream" in the BMW world. A turbo setup a with fuel injection is more common than a carb setup. The M20 is already setup with EFI so complications maybe less than an M10. I guess what it really comes down to is "Bang for the buck". Im sure you already know that it would probably take more money to get power from an M10 than an M20. M20's are pretty strong, Ive been in a car with just a chip and was impressed. Hope this helps somewhat, try to contact Kris (oldskoolbmw) , or try
www.2002faq.com ,a lot of real tech info there, if you dont mind the forum setup there.