How much of the damage to valves/valve seats is inherent in the fuel and how much is due to badly tuned LPG systems? If you run an engine badly tuned on petrol it's going to hurt stuff too.
A mixer system can be made to work well. They're easier to set up than the equivalent petrol fuel systems (carburetors). The problem is they're usually not set up very well. Often you're fighting poor engineering - up until a few years ago every LPG system could be engineered for each individual installation - as well as poor implementation.
On a NA engine on LPG with a bad tune, if it backfires through the intake, you break the airbox and not usually anything worse. If it happens on a S/C engine the S/C may be pushed backwards. Not so good.
On a purely monetary basis, assuming nothing is being damaged as ^, vapour injection doesn't make as much sense as a cheaper mixer system (there's an epic thread in VT-VX on why). Liquid phase injection is worse. On a performance basis there's probably a better case to make for them.
If you are running a NA V6 (not what the OP was asking I know) using LPG injection means losing the possibility of using the "off the shelf" ignition timing map for LPG. Of course, you can have a custom map done.
Dual fuel has some advantages in that you have a greater range, should you need it, and there's a cost penalty in removing the petrol system, without much benefit other than losing some weight.
If you are prepared to make internal engine mods or maybe boost on a stock compression ratio, I think dual fuel E85-LPG would be interesting as many of the modifications you would make over an engine optimised for operating on petrol are the same.
