Well, we all come at things through the lens of our own experiences, so what I'm about to say comes from my own personal experiences with a number of different grinders that use tension springs versus compression springs for belt tensioning.
Its all about power of the belt tension, and leverage. All of the tension spring grinders I've come across have the tension spring connected to a short nub of the ilder arm that sticks out to the rear of its mounting point, and the other end to the lower part of the grinder frame.
From the standpoint of leverage, this is very weak as far as the force it exerts on the belt tension. The tesnion spring must overcome the leverage disadvantage plus keep the belt tight. Its just a loosing situation compared to a compression spring that is mounted at, or near the spot in the idler are where the tracking/idler wheel is attached. The biggest issue most folks have with tension spring belt tension is the belt "wandering" more and more as grinding pressure is increased.
You can always put a heavier tension spring on a grinder, but by the time you have a large/strong enough spring to match what a compression spring provides, the idler arm often "jumps" straight up when you remove a belt. I personally know of one guy who lost a tooth when he released the belt after installing a heavy tension spring, and lost his grip on the idler arm handle..... and got smacked in the mouth by the idler arm handle.
On a lot of the "Homemade" grinders (and/or those with assembly required), there is often some "slop" in the connection between the frame and the idler arm (must be enough to allow the arm to move, but not so much as to allow side to side play)....this causes a tension spring to "pull" the idler/tracking wheel slightly out of alignment with the rest of the wheels. Of course how much depends on the distanct the idler wheel is from the idler arm connection point......and how the tension spring is connected (in a straight line of one end out of line with the other). With a compression spring mounted as close as possible inline with the connection point of the idler wheel, those issues are a non-factor.
Theres a whole other set of variables when it comes to the using the "lift cylinders" for belt tensioning. But if some thought is put into how they work, and they are mounted accordingly, they are very nice. I feel the most important thing with lift cylinders used for belt tensioning is to ensure they are mounted with the "rod" facing downward, to protect the seals...... I had two lift cylinders fail in two weeks, that were mounted with the rod up, due to grit getting into, and wreaking the seals. I've since gone back to compression springs on all of my machines, simply because they require no maintenance, and are far less costly then replacing lift cylinders.
Finally, personally I feel that with a grinder that has a sliding tooling arm, such as the KMG, EERF, etc., the combination of a compression spring, used in conjunction with the tooling arm being capable of "sliding" back and forth offers more versatility, especially when it comes to "slack belt" grinding situations...... I just find the ability to "soften" or "stiffen" the belt much simpler/easier with the compression spring/tooling arm then with other tensioning systems.
