Well, I'm against the idea of .22 conversions. Why? Yes, they do allow you to practice with .22 using the same gun you'll shoot center fire with. However even the quickest switch over is going to take near a minute and in many cases longer. That kills any sense of continuity you'll have compared to just fluidly laying down one gun and magazine and picking up the other and loading and shooting. Not to mention that for my part I found that one or two magazines of .22, then a magazine of 9mm, then back to one or two of .22, then a mag of 9mm, etc worked the best for me. With that in mind the frequent top end change overs would get really old really fast. So for me the choice was clearly two complete guns. And when you consider that there's many really nice .22 guns that cost no more than a conversion kit the choice is not one for saving money but rather it's one of convienience and personal choice.
Which one to get if you decide to go with a separate .22? To make it a more effective practice companion to your center fire it should be close in grip shape and angle. So unless you get an old P08 Luger this lets out the Ruger .22's and early target shooting Hi Standards other than the Ruger 22/45. .22's with a grip similar to most of the regular center fire guns in addition to the 22/45 are the Browning Buckmark, S&W 22A, S&W 422 , S&W 41, Berreta 87, Tanfoglio Force22, Sig 226 in .22 and others of similar style and grip angle.
By the way, not everyone gets bored with .22. We've got some matches around my area that .22 is elligable to compete with and when shooting with a timer behind your head .22 isn't such a bad option and suddenly becomes fun in its own right. But I'll admit that for purely punching paper it's a learning tool instead of a fun thing for many.