Personally, I have got a very good experience with the Siemens/Infineon 80C535 and 80C517A in a Peruvian University. I am currently preparing 2 student projects which will show the use the 517A to control a solar UPS and on an agricultural irrigation control project.
The MPS51 family is very wide and this choice was dictated on the availability (and costs), the free software support,and the hardware features: the // ports number available on the chip and its 12 Channels x 10 bits A/D converter.

Best regards and good luck

JF Duhamel