Any shorter wheelbase loco will be better at tighter turns. That means that x-4-x or x-6-x are generally better than (for example) x-8-x or x-10-x or even x-6-6-x. Older steamers used to be made with "blind" drivers - that is the middle set (or two) of driving wheels had no flanges so the wheels were free to ride out over the rails in the tighest turns.
I have a Proto2000 0-6-0 that I have equipped with a Soundtraxx DSD-LL080LC decoder. This is the "plug n play" decoder that is designed for the Proto2000 0-8-0 switcher, but it will work with the smaller engine too. Sounds great!
I also have a Bachman Spectrum 4-6-0 (no sound yet) that works well on 18" radius.
Remember too that long rolling stock, like heavyweight passenger cars, may have some problems and/or appearance issues on 18" radius.
Andrew