Thanks very much for the nice comments! 

The Lionheart pannier has a Zimo decoder with Lionheart's own sound 'project' (I think that's what you call it).  It is amazingly good, with great sensitivity to the load on the loco, very realistic sounds both under power and when coasting.  I can't really imagine the sound being better - it's really that good.  The volume levels of each of the various sounds are quite easy to adjust to taste and I found I needed to turn the coal shovelling sound down a lot. As supplied it sounded like a demolition gang working in a corrugated iron workshop, but now it sounds just right to me.
My Tower Brass 4574 and 14xx were retro fitted with Zimo Decoders by Coastal DCC.  This wasn't entirely a happy experience - when the locos were initially returned to me, after a long period of patient waiting, the 4575 would only operate at maximum speed and the sound bore no relation to the speed, and the 14xx would not move at all.  Telephone calls to Coastal DCC got the 14xx running after some re-setting.  It now is very controllable, but I'm not convinced by some of the sounds, the chuffing sound is very "whistly" and having watched a lot of 14xx's in steam on videos, I can't really say it sounds right.  The other sounds are ok though.
The 4575 was returned to Coastal DCC and came back working but with the quartering out of adjustment, so it wouldn't run smoothly.  I was able to fix that easily enough, and the operation and control is now superb.  The sound is good, but I don't think it's nearly as convincing as the Lionheart pannier.  I can live with the sounds I've got from the 14xx and 4575, as the locos both work very well and they sound acceptable to good.  Coastal DCC have a good reputation and I don't wish to tarnish it but I can't personally recommend them for retro fitting "0" gauge brass locos - they are certainly knowledgeable and were very helpful in recommending a suitable controller (NCE Powercab) for my layout.  I should say the wiring and installation of the decoders and speakers in the locos was done very neatly; much more so than I could have achieved.
I'm reluctant to give a sweeping judgement on any particular Decoder etc as my knowledge is limited, but I am very pleased with the result I have now.  
Did I mention the quality of control before?  All the locos will now move reliably at the speed of  a lazy snail.  They simply don't stall at all, presumably thanks to the stay alive capacitors they are fitted with and I don't think I've cleaned the track since August.