I've been testing two different DCC decoders in a variety of locomotives. Results are interesting, and I would welcome comments.
Tests were all "on bench" rather than "on layout", using a Sprog device and JMRI (DecoderPro) for changing settings and driving throttle.
Decoder 1 - CT Electronik DCX74D
This is the "short fat" version of the DCX74, and before the announcement of the DCX75 in January 2008, the smallest volume DCC decoder.
Out of box settings; works incredibly well on Faulhaber coreless motors (816, 1016 in small diesel shunters, ~35:1 gearing), with locos running really dead slow. Smooth all the way up the speed range. Some fiddling of acceleration/deceleration can alter things, as can the speed curve, though only really necessary if wanting to speed-match two locomotives for double-heading.
After fiddling of PWM setting to "141" (manual suggests this is lowest permitted), the buzzing on Mashima open-framed motor is removed (10-16 version with ~35:1 worm reduction), and gives nice slow running with minimal cogging at a crawl. Further changes to CV's may help.
Again, with the changed PWM setting, the DCX74 runs my High Level kit built shunters (4mm P4 standards, Mashima flat-can motor, 100:1 gearboxes) nicely. Lower starting speed than the Lenz, though somewhat academic on a loco with 100:1 gearbox. The hunting I saw on earlier bench testing with the Sprog was not present when I ran via a Bachmann E-Z controller, so I put that down to an artifact of the Sprog and bench tests. DCX74 gets fairly warm running these locos, I wonder if the higher current rating of the new DCX75 may cooler ?
Decoder 2: Lenz Silver Mini.
This is one of the most commonly mentioned "quality" decoders in the UK market.
Coreless motors - its is RUBBISH. Motor will not turn at a crawl, its either "off" or "modest walking pace". I have failed to find a setting to alter this, and various web searches haven't turned up the appropriate settings. Lenz manuals are opaque on the subject of motor type; they say it exists but give no clues as to what the values might achieve.
Small iron cored, such as Mashima open framed, and an "Poole built" Farish 5-pole. Reasonably good out of the box, though lowest speed is higher than the DCX. Again unable to find any improvement by changing motor values for the Silver.
Small/medium can motors, such as the Mashima's in my 4mm scale shunters. Good control, nice speed range and will work slowly. Some tweaking of acceleration/deceleration and speed curve values may be useful.
If using small coreless motors, do not use a Lenz Silver Mini, instead get a DCX decoder. (You probably have to use a DCX anyway as its half the size of the Lenz!)
If using small/medium can motors with high gear ratios, either Lenz Silver or DCX74 with tweaked CV setting.
If using small open framed motors, use Lenz Silver if not into CV tweaking. With tweaking DCX74 gives better control.
Comments and observations from others wanted.