Shanewomer:
Let's take it a little slower this time, regarding your questions:
1)First, although the S&W K-frames aren't as strong as the Ruger SPSP101, Service-Six, Security-Six, Speed-Six, GP100, it is still plenty strong! If you shoot mostly .38's(Or, .38+P's)and just a moderate amount of .357 magnum ammo, you will not be harming your K-frame! Just don't shoot too many 125 grain .357 magnum's-as these will erode and damage the forcing cone(In due time!)and, the gun will shoot loose(Sooner rather than later)! Shoot the heavier weight .357 magnum ammo and you will be fine!
2)On your K-frame, look at the forcing cone(In the 6-O-clock position)with a magnafying lense in good lighting. What you will be looking for, is a fine crack in this location-as there is a flat spot(By design)in this position, that makes the wall thickness thinner-and, therefore the reason for the cracking problem! Also look for forcing cone erosion? The edges of the forcing cone should be sharp and not look worn away?
3)The difference between a Ruger Service-Six and a GP100, is that this gun(Along with it's brethren, the Security-Six and the Speed-Six)is very close in size to an S&W K-frame and is easier to handle, easier to carry, and is easier to conceal! Although the GP100 is a marginally stronger revolver(By virtue of it's larger frame size and having the lock built into the front of the crane, the service-Six(And it's brethren)were also super strong guns(Especially for their medium frame size!)and therefore strength was never a problem with these revolvers(In the same sense as it was for the S&W K-frames)! Both the Service-Six and the Speed-Six, have "Fixed" sights-wheras the Security-Six has the adjustable sights. The Service-Six and Speed-six, differ only with the speed-Six having a round-butt grip frame(With the Service-Six having a square butt grip frame)and, the Security-Six has the same square butt, grip frame, as the Service-Six(Also named the Police Service-Six). Both the Service-Six(And, the Speed-Six)came in barrel lengths of 2-3/4". 3.0"(More rare), and 4.0". The Security-Six, also came in these barrel lengths-but also, in the 6" barrel length! These are excellent revolvers! They hold 6-rounds! The GP100 came in both the "Fixed" sights and with "Adjustable" sights(In barrel lengths of 3" and 4"(With "Fixed sights)and in 4" and 6" with "Adjustable sights! All of these guns mentioned here, came in blued or stainless! The available calibers(On production guns)were .38special & .357 magnum, and some were in 9MM. The Security-Six(Except for some very early models!)was only available in .357 magnum caliber!
4)My recommendation: If you want a Ruger-then, my pick would be either a Service-Six or speed-Six because of having the fixed sights and because they are the same approximate weight(And size)of an S&W K-frame(In any equivalent barrel length)! Although the "Fixed" sight 3.0" GP100 is a good gun, to me, it is too bulky for carrying it! If you want a smaller Ruger(And, if having 5-shots, doesn't bother you?)then, the SP101 is also a good, strong, reliable gun(Except that the trigger isn't quite as good as the "Six-Series revolvers or the newer GP100's!)! Lastly, If you want to get a 3" S&W K-fame-this is still a nice revolver(With a great trigger, in the older models!)and will be strong enough(As long as you don't try to kill it by shooting "Hot" .357 magnum loads, continuously(This is the key word here!)!
5)Since I happen to own a Security-Six, Service-Six, and a Speed-Six(All with the 2-3/4" barrel length)Ihave included the following pictures of them for you to view:
Service-Six(.357)square butt:
Speed-Six(.357)round butt:
Service-Six and Speed-Six shown together(Note that the Speed-Six, is a later gun, that had the dimple in the right side recoil shield):
Security-Six(.357)square butt: