If the frame has a long neck, the range of possible adjustment for your seat-post will be increased. If you’re a beginner, you might benefit from playing around with seat height. SIF (seat in front) and freestyle riding, for example, use a higher seat, while SI (seat in/under) and flatland riding use a shorter seat. I did a lot of experimenting with seat height early on, and I was glad my Equinox frame allowed me a larger range of adjustment.
On a side note, I wish the neck on my 26" Oracle frame were a tad longer; the seat post that is compatible with the Shadow handle comes in one size, and to get the seat high enough, I’m raising the post past the minimum insertion point. So far, so good.
If you think you might be letting others try your unicycle, it’s nice to have a range of adjustment. The Equinox long-neck frame, however, is too tall for most kids, even with the seat-post fully inserted into the frame. I assume you’re tall enough for the Equinox.
My repertoire doesn’t include any techniques causing the seat-post clamp to catch on any part of my body. Some riders have referred to this issue, but I’m guessing for the large majority of riders, it’s not an issue. Same for me with the Equinox’s square crown. Maybe various leg wrapping techniques create an issue with the crown. However, I have gotten more utility out of having the square crown, particularly for one-footed techniques.
Lots of riders trash the Cyko-Lite tire (stock with the Equinox) on the forums. I think these are either very advanced riders, or riders performing a lot of gapping with lower tire pressure. The Cyko-Lite is long-wearing and it doesn’t totally suck for a variety of riding conditions.
Regarding the 25.4mm (Nimbus) vs. 27.1mm (KH/Impact) diameter seat posts: The only problems I’ve had with the 25.4mm seat posts on my Equinox and Oracle…involved the failure at the top of the seat post where it forms a “T” and attaches to the seat…which is not really caused by diameter.
I say go with the Equinox!