Hi Hue,
Since this is your first post and you just subscribed let me be the first to say welcome to this community, its generally a nice place and you can get some great info here considering there are a lot of experienced divers.
The market demand commands the product line most of the time, not just what we think is better. In theory an integrated shaft guide serves the purpose of guiding the shaft straight during the shot. This is particularly important in a gun with offset bands, such as loop bands with a diagonal pull when the shaft would otherwise be pulled down onto the barrel of the gun.
In practicality this makes perfect sense but when Marco Bardi and other top team Omer collaborators did some tests and I did mine 3-4 years ago we noticed that on a 90-110cm guns there wasn't a noticeable increase in accuracy with the stick on rail so we opted not to install one stock on the guns. There were some scratches from the shaft and that was it. I am convinced a rail offers no increase in precision in a 90cm gun nor a 100 or 110. However, because it adds roughly 20% stiffness to the barrel and to satisfy the market demand Omer started producing extruded rail barrels last year.
The argument changes in guns over 110cm where barrel flex can be a contributing factor to loss in accuracy as the shaft error is amplified by the flex in the barrel. In other words if you overpowered your 120 aluminum round barrel gun, your barrel may flex/bow and so would the shaft. Not to mention the "average" European speargun handle trigger tension increases with the use of two bands further augmenting inprecision by the likelyhood of flinching during the shot. If you keep it simple and use the gun as it's designed with a single power band, regardless of whether it has a rail or not, it will always be more accurate.
For this reason, and to find a better replacement to carbon fiber, we went to the new technology of Hydroforming in our HF models....they're even stiffer than our signature model Cobra teak guns and are extremely light. The possibilities of speargun barrel design with hydroforming technology are infinite. We can make a gun and shape it like a surfboard or any way we want to increase tracking and maneuvering, add ballast where it's needed quite easily, and the structure is lighter and stiffer than any others including carbon fiber in the same dimension.
We're working on a 130cm model for next year which will be awesome. Sorry I got carried away
v To answer your question between a Cayman 90 which has a rail and an Exc. 90 quite simply, if you're just going to use a single band you won't be able to tell the difference in power and accuracy between one or the other. If you decide to use two bands the Cayman is obviously better because it has the most sensitive trigger mechanism I know of unaltered by band load and the integratdd rail makes the barrel 20% stiffer.
Azapa, US discount online dealers like
Buy scuba gear, scuba diving and snorkeling equipment, and everything underwater from LeisurePro.com just to site an example are as cheap for us here in the US as the guys in Europe when you factor in shipping. While places like Leisurepro are the best solution for someone who doesn't have a good local Omer dealer in the US, it may not be beneficial to you in Chile where I feel you're still better off befriending the local Omer dealer who also has to try to survive. Three months after your purchase you're still dealing with an issue of a bent shaft that could have been his problem not yours.
Ciao & clear waters, Mark