If you think you can use patterns like Variations and Fills on an arranger, you are sadly mistaken. Yes, you can use them to make accompaniment, but the whole paradigm is utterly different to how an arranger works. An arranger is all about spontaneity, about being able to do ANYTHING the second you decide to do it, a loop station is something you decide IN ADVANCE what you want it to do. You can't drop a fill in wherever you want it, you HAVE to cue it up a bar in advance. You can't preset where it goes to, you HAVE to input it's destination by hand... Perhaps not important in a studio setting, where you don't need it to be realtime, but it makes a HUGE difference when trying to play live. Trust me on this one (I've tried to do it!), you might like to create a song on a MoXS at home, but take it to church, and decide to mess with the structure live, you are going to be cursing it's obstinacy probably more than how you feel about me..!

As to song creation, well, there's no right and wrong way... just whatever works! But some ways are easier to do than others. I've used your system, I've used others. To be honest, when simply working on arrangement and structure, I tend to do it the old fashioned way and simply PLAY it, sans backing, until I hear it flow the way I want it to, then start to add the fluff. I find too much fluff at the start tends to color my impression too early. But there's no WRONG way, as long as it works.

And, sorry, but a ten year old WS isn't going to impress you very much in the ability to create live arrangements. And current WS chord following abilities are at LEAST ten years out of date. If the ability to voice complex chords correctly, to have bass lines follow inversions, and all the other things we take for granted on an arranger are in the least bit important to you, having them completely missing even on the TOTL WS's is going to be a hassle. You wouldn't trade your PA in for a ten year old arranger, you'd miss WAY too much stuff. Sadly, you are going to HAVE to miss that stuff if you try playing on a loopstation WS.

Neither is an adequate substitute for the other, yet, IMO. That's why I have both, and USE both. Trust me, if I felt one of them could cover BOTH bases equally well, I would sell the other in a flash.