I've owned both. In my opinion the Plus version is the only way to go. The base model/one without adjustable voltage simply does a very poor job of marking consistently. If the blade your marking is VERY small and THIN (as in a slipjoint blade) you can get a decent mark most of the time with the base model. If the mass is anything much larger, it simply doesn't have the power for a deep, clean mark.
Electro/chem etching is one of those things that everybody does slightly different to achieve the results they seek, so there is always a bit of a learning curve when you first start using one.
Hi Rick. The difference is that the Personalizer Plus has adjustable power settings. The power on the Personalizer is fixed. I just recently purchased the Plus model. Still playing with it. Not sure if it's worth the extra but I like having the option of being able to "dial" in the power. A thread and example here: https://knifedogs.com/threads/new-etcher-and-stencils.46735/#post-354094
I have the old model with no adjustment. I personally haven't had any problem with it at all, but I also practiced scrap steel until I got my times down. With the non-adjustable unit your results will come from a consistent approach both during the etching and then the marking. I do it the same way every time and my results are the same every time.