From an installation perspective the X5529A-R6 (swift) rail kit is much easier to work with in a single person install. They also ensure proper spacing so if you leave gaps for future equipment installs, there will be sufficient space available.
The standard rail kit (X5526A-R6) that NetApp seems to ship as their standard can twist during install and just enough to make it difficult or impossible to get equipment installed in those spaces left. During my install days, I always wanted to rack bottom up to make sure I didn't get bit as was the case a time or two.
I honestly don't understand why NetApp doesn't adobt the swift as their standard except for it probabally costs more or their margins are not as good.
A lot of sales people are not aware of them as they love defaults and send the longest cables possible which make the installer's job a nightmare and installations look like crap. This is also due to NetApp's inflexible ordering system.