First things first... that lift is awesome...
Now onto the question - I recently posted this same question and was met with a bunch of opinions both for the 15mm and 21mm machines. I personally own a ton of 5.5" pads, so I did not want to buy something that did not allow me to use what I already had. I was originally dead set on the 15mm machine as I tend to work on sporty coupes to sedans, rarely an SUV or truck - so I thought the 21 was not necessary.
At the last minute, after an overwhelming majority opinion to get the 21 + 5" BP, I decided to not get the 15 and instead go with the 21. This decision was based on the fact that I did not want to limit myself to only using 5.5" pads in the future. The option to jump up to 6.5" pads seems like a great reason to invest in the LHR21 vs. the LHR15.
I have used my Rupes on 1 vehicle so far using my 5.5" pads (
CLICK HERE for thread) and I liked the machine. I will warn you though, as Roshan (ihaveacamaro) already posted in his thread - this machine is not a miracle machine that will simply cure all defects in 1 pass, as much of the hype surrounding these new machines can lead some people to believe. Do not get me wrong - it was great, no denying that, but it still takes time. Slow arm movement and proper technique are key. The reduced amount of pressure required and minimal machine vibration was a nice advantage, as I found myself less fatigued after an 11 hour job.
I found that it did indeed take less time and effort to correct even some pretty serious defects. The 50/50 shot below shows the original condition of my fiance's new DD compared to what the LHR21 did with an orange pad and FG400 in just 4 passes (2 horizontal and 2 vertical) with light to moderate pressure and slow arm speed. Now I obviously have not used the LHR15 for comparison, so I do not know what the difference may be in terms of time saved - I would assume it is not drastic.
I will be using it this weekend on an Acura TSX and will be sure to post the results in the Show N' Shine section once completed.
Overall, I think the general consensus has been that you will be satisfied with either machine, as they are both outstanding... but if you can splurge for the 21 + 5" BP, why not go for it?
Hope I helped in some way!
-Zach