Conversion unit.
I sold off my Ruger MKII & 22-45 years ago with no regrets, they did nothing to improve my shooting for IPSC as they did not match the feel or controls of my centerfire guns.
Tactical/practical/defensive shooting is more than rounds down range or trigger control, it's reloading, working the safety, mag release,, instinctively picking up the sights, draw stroke and a few other variables that come from making the gun an extension of your body.
If you buy a plinker and train a lot you will master accuracy and basic trigger control. You will still have to relearn it to suit the centerfire you intend to shoot with, and you will have to unlearn the controls to relearn the new set, even if you are a good shooter.
I was on the Australian IPSC team many years ago. I shot mainly with a Glock, sometimes with a Beretta. I had a draw stroke that would naturally knock off the safety on the Beretta and was suitable for both guns. When I shot a match with a 1911 I drew and failed to disengage the safety - under pressure I had reverted to what I trained for. This is a good example of why I think a conversion kit is a good idea - with most it will duplicate the factory controls and sights.
My Beretta practise kit did improve my shooting with the Beretta.
My S&W Model 18 in .22lr certainly improved my shooting with My Model 66 in .357 Magnum.
Conversion kits weren't available when I bought my Glock 17, so 20,000 rounds of 9mm went down the pipe. A conversion kit would have been much cheaper.
Oh yeah, don't buy a Ceiner conversion. His reputation for poor quality control and non existent service goes back a decade and he has now been charged with fraud, so may not be in the gun business much longer.