I have a 20l and love it. I could have brought either but chose the smaller one. Why, because I live in a place whear space is at a premium, I am not limited to a particular place for my brew in (I some time brew in the laundry, shed or kitchen depending on the day) I can take it on the road, my kegs are 19.5 l, I have one 30 l fermenter and two 25 l buckets that all fit together, there are lots of other volume related things because it is such an easy size to get. We are also looking at buying a unit so 15amp points may not be able to be had.
Why I looked at the BM50. Bigger batches which meant less brewing. I did like the idea of being able to brew bigger beers by making less at a time but still finishing with a decent amount for my day, but I don't drink that much big beer ( in fact I have just made my second beer over 6.5% after five years of serious brewing). It came with the need to continue buying larger equipment ( taking up more space) because I like to chill my worth then pitch as I do not like having cubes hanging around, the need to have a 15 amp power point and i don't mind the extra brew days meant that the BM50 was not for me.
You cannot ask the question which one is for me and get the right answer because what one man like another hates. Instead work out what your need /wants are and seek peoples thoughts on it.
Hope this helps.