I have an STec 30 with a GPSS steering unit connected to a Narco HSI and a Garmin 430. I also have the alt unit. It was added as a retrofit by me a couple of years a go.
I did not have the panel space for the 50 and the 30 was a perfect slot in replacement for the TC which was on the fritz anyway.
I think that is the main reason people tend to go for a 30 over a 50. Performance wise they are the same.
I find the 30 to be simple, totally reliable and handles turbulance very well. with the GPSS unit it will fly a complete IAP, SID, STAR etc.
Can't find a single fault with it.