I don't know about the Craftsman one but all the other discussed have platens. They can be used or not. You can work over them or on the slack belt section adjacent to them or turn them around and have the whole belt slack.
The
Harbor Freight 1x30" belt sander is around $40. It has a platen that you can work over for flat grinds or chose not to for convex grinds. It sounds like it would do the job. It runs a little fast for some folks generating more heat and making your mistakes bigger. Also some have reported tracking problems with the belts. This is very inexpensive, fast, small, light, and portable for a belt grinder. Many professional sharpeners use these for mobile use because they are small, portable and fast.
The Delta and Grizzly are 1x42" sanders and both have platens the run roughly half way between the wheels. You can use the slack belt for convexing or work over the platen for flat grinds. They are both bigger and heavier then the HF.
Both are better then the Harbor Freight 1x30" in quality and I think they both run slower stock. Since the motor is not direct drive to the bottom wheel, but belt drive) you can modify them with step pulleys and slow the belt down. That is nice for sharpening and not generating too much heat. You have to remove and lose the sanding disk plate to do the modification. If you go for the modification both will be around $200 when finished.
If you lean a little more toward the knife making side I think the Kalamazoo has different diameter wheels available which is nice for working different radius grinds. The one I am thinking of is about $230.
Both of the ones you linked are quit a bit bigger then any of these. The Craftsmen 1x42 says it has a belt speed of 4400 fpm. I run my delta at around 800 fpm for sharpening and maybe at 1600 FPM for reprofiling with coarser grit belts and get virtually no heat build up, no quenching required. It is very precise and manageable with the step down pulleys. It worked well without the pulleys but was still much slower then this craftsmen. IMHO 4400 FPM is to fast for sharpening for most folks especially when learning. It also generates a lot of heat with the finer grits. The Craftsmen 2x42 looks like it would be hard to change belts and work with all the shrouds (covers). I can't tell about the platen or if you can take all that crap off or not.
The higher speed would be fine for large metal removal when making knives. Some of the
pro belt grinders or this one go fast but are also variable speed and can be slowed down. They also have the option of changing contact wheels to different diameters for grinding radii over the wheel, but then you are getting into the $1000 range for the grinder and another $1000 + for the motor and speed control so you are getting into the $2000 and up stuff.
I like my
Delta SA180 but if I was getting one now I would get the
Grizzly because the Delta is discontinued (you can still get them) and I think the Grizzly is made slightly better (it looks the same). The step down pulleys are not necessary but really make it nice to work with. My main complaint would be you can't do much work over the contact wheels because they are not exposed very much. but to step up is to go into the ~ $2000 and up stuff above. The
Kalamazoo 1x42 might be an option after taking off the guards but I hear complaints about things that need modification on that one and I don't know about speed.
Here is a guy sharpening on that one. He says it is a Baldor but I have heard that is just the motor because that is the only thing that has a name on it (it's a Kalamazoo). Anyway it is the same as the Kalamazoo and the sharpening technique would be about the same with any belt sander.
BTW I use more girts then he does, going for a polish, and then strop on loaded leather belts like he does. I also don't get sparks like he does. This thing is going pretty fast. The only way to step it down would be with a variable speed motor (problem like the Harbor freight) or a slower spec'd. motor.
