I did some testing recently of the "traditional" in-line strip-loaders in comparison to the Zeta 6-Pak staggered style strip-loaders. A few conclusions:
1. If you only carry one, the in-lines are more compact in a pants pocket. The Zeta-6 is fairly bulky, and the staggered design is not so pocket-friendly, getting easily tangled up and potentially losing a round on drawing it.
2. If carrying TWO loaders, the Zeta-6 is designed to nestle into each other, creating a compact and secure unit - albeit somewhat bulky. The advantage though is that the Zeta-6 presents itself in the same way every time. Two in-line loaders in a pocket tend to tangle up with each other, and not necessarily present themselves at the same orientation every time.
3. Speed. The single in-line loaders are predictable: so long as you find set the tab into your pinky or palm, they work the same way every time, and for every gun too - 5-shot, 6-shot, 7-shot, etc.
The Zeta-6 in the 5-shot version is just plain odd, and due to the small cylinder diameter, trying to load 2+3 or 3+2 is difficult. Aligning three rounds at a time requires a lot of looking at the cylinder. If you load 2+2+1 with it, it is actually slower than the in-line type. For a 5-shot gun I would recommend the in-line type (Tuff Products makes a 5-round Quick-Strip which is more compact).
For a 6-round cylinder, the Zeta-6 6-shot version is more predictable - it doesn't matter which end you present first, it's always 3+3 loading. The larger 6-round cylinder makes aligning three rounds easier. With practice, it can be about 1 second faster than the in-line type, as it is only two moves to reload, instead of three. Practice is essential because aligning three rounds at a time is not as easy or positive as only two, and also requires a full 180 turn of the cylinder after the first three are loaded.
For what it's worth, I use a 5-round in-line Quick-Strip for 5-round .38/.357 guns, and the Zeta-6 staggered style for the S&W K-frame or Colt 38/357 (carried as a deuce).
Your only choice in other calibres (.32, .41, .44, .45) are the in-line styles.