Having owned an Electro, these are the things that I have to say:
1. I haven't had an XK3, but from the demos, it seems the XK3 beats the Electros leslie sim to me. One thing that I didn't like about the Electro (and a lot of organ clones) is that they use foldback for the bass. This means that the normal deepness you hear from an organ, you won't and can't hear from many clones. The XK3 allows you to choose whether you want foldback or not.
2. The leslie sim was too shrill (high) for me on the Electro.
3. The Electro doesn't allow you to tweak the leslie sim, while most of the other clones do. This will let you get a better simulation or make the organ sound like different hammond models.
4. It has buttons instead of drawbars.
The only things that I liked about the Electro were:
1. It was light
2. It had good electric pianos
Now, having said all that, if you decide to buy an external leslie or leslie simulator, disregard all my cons (except the drawbuttons ofcourse). With an external leslie simulator, almost all the organ clones are equal in sound, so you could get a cheaper one.
I wonder who told you that the Electro was better, since everyone knows the XK3 beats the Electro, hands down!