The difference is in the motor FET parts, which are (to my dismay) sandwiched and soldered between the two boards. The FETs on the 10amp version are beefier but also do tend to run a little hotter than those on the 6amp which is why the 10 is issued with a cooling fan. In future runs I want to move to a universal motor controller part and add a connector set between the top and bottom board, to improve serviceability.
I think, if you're running the 6amp decoders now and they haven't given you any trouble, you'd be fine just by adding the cooling fan to your 6amp decoder. You can do this one of two ways:
We support a specific light port in Large Scale firmware for "Decoder Cooling Fan", it's Port 11 in all Titan-ET (Q3), FX-LS firmware. This will turn the fan on at all times when the decoder is powered up.
If you don't want to sacrifice a light port you can wire it between J3 pins 10 and 11 (ground/+5). The thing to be aware of here is; the fan will run any time the engine is on a live track (whether powered up and making noise or not). This also means it will turn on and off with every packet sent by the Quantum Programmer and as such screw up a download and interrupt service mode programming. Make sure you have a means of disconnecting it if you plan to use the Quantum Programmer.
If the decoder has been acting strangely after/while running a long train you can monitor the decoder temperature using the F10 status report feature, all you have to do is check "Engine Temperature(degrees C)" under feature config->status
in Quantum CV Manager or set CV55.178.0=8.
I hope this has answered more questions than it's created!