When I first started working with J, I bought one of the Sonotrax. It's okay, but I wasn't too impressed with the volume as it didn't get all that loud IMO so I exchanged it for a Huntleigh and have been happy with it. J always buys the Nicolet dopplers... I like the probe in a lot of ways- it's shorter and wider and has a broad beam which can make finding FHT a little easier. Otoh, I tend to end up with gel on my hands more often (I find gel annoyingly sticky).
One of the reasons I ultimately went with the Huntleigh is because the probe clicks into the side of the doppler so is protected when not in use whereas the Nicolet doppler head is exposed all of the time.
A sonotrax is certainly a good starter doppler, you could try out the other midwives' dopplers and get a feel for what you like before you spend more. Have fun shopping!
eta: I see a lot of other brands on eBay now... I don't really know much about them. I've only ever used the Nicolet, Huntleigh, Sonotrax and LifeDop.