I've got an Olivia's Castle and a Howes MN; The Howes sound is fantastic. Also have a Howes A1, also fantastic.
I'm not a fan of Olivia's myself; When I got my castle the wheels were completely out of synch, I found the chuffs weak sounding, and whistles are preset tones/quills. My biggest issue, aside from the whistles, is the wheels tend to be out of synch a lot when people buy from them. It seems to be fairly common. I won't lie though, if it weren't for the wheel synch problems Olivia's does do okay steam sounds. However they share a lot between models and aren't always accurate to the model because they carry-over so much.
Go Howes. Or Locoman. Won't regret it.