I purchased two clips to hold my new GSM Treo 650 on my belt. Since the Treo 650 is bigger than my old phone, I purchased the horizontal version of the Proclip. The Seidio only comes in a swivel version. They differ in both in the belt attachment and the main clip that holds the Treo. Because of the different bodies, there's a difference in how you take the Treo out and put it in.

Belt Attachment

I once purchased a case with a clip that was not spring-loaded and lost the device I was carrying within 24 hours. Never again. Both of these clips have a heavy spring-loaded clip, unlikely to fall off my belt in the normal course of squeezing into cramped airplane seats.

I have a worn a cell phone on the left side of my belt for years, so I was dismayed to find that the Proclip was designed to be worn on the right side. A swivel attachment is included, but it makes the Proclip stick out much farther. The fixed clip is contoured and about 8mm to 12mm thick, while the swivel clip is straight, clunky, and 20mm thick.

By contrast, the Seidio is swivel-only and the swivel "clicks" into place at seven positions, 0 degrees (vertical), 30 degrees, 60 degrees, 90 degrees (horizontal), 120 degrees, 150 degrees, and 180 degrees (horizontal). This makes it easy to wear the Seidio on the left or right side.

Body

Both clips are designed to hold the Treo with its display facing the body of the clip, affording some protection against scratches.

The body of the Proclip is diamond shaped. In photos on the website, this appealed to me because it looked smaller and less obtrusive the big rectangle of the Seidio. The problem is that the diamond doesn't fully cover the face of the Treo 650 and it fits too loosely for my taste. The Proclip allows the Treo to rattle around a little bit. Not much, but enough for me to be concerned about how it will fit in a year.

The body of the Seidio looks clumsier in photos, but offers practical protection for the face of the Treo and does not seem too big in use. The Treo fits into it more snugly than it fits the Proclip.

Both clips allow headphones to be attached before the Treo is inserted into the clip (or after, for that matter).

Out and In

Putting the Treo in is easy with either clip but removing it is easier with the Seidio. Both clips require that one end go in first. The Seidio requires that the bottom of the Treo go in first, while the Proclip requires that the top go in first (else it will pop the SD card).

In both cases, you press the Treo into the clip and it snaps into place. Then you have to press on a piece of plastic, a tab, to release it. Here is a major difference between these two clips. It's natural to use your forefinger on the Seidio tab while holding the Treo between your thumb and other fingers, but the Proclip instruction sheet suggests using your ring finger, middle finger, or pinky, while holding the Treo between thumb and other fingers. I was too clumsy to do this successfully, so I finally just started using my forefinger and taking the Treo out upside down, then turning it right side up.

Conclusion

The Proclip is more flimsy and offers less protection for the Treo and requires that you wear it on your right side and be dexterous with your pinky. I'm keeping the Seidio.

p.s. Proclip mainly does car mounts and this is compatible with their car mounts, but it leaves the Treo facing away from you. I think the traditional car-mount position is with the display facing toward you so you can look at phone numbers or play lists while driving like a maniac. Come to think of it, maybe facing away from you is a feature!