Given thoss two choices, as asked, I don't think there would be much difference, as both should have power to spare when it comes to setting-off the primers.

Years ago, I shot Police Pistol Competition (PPC, also called Police Pistol Combat), and there were many gunsmiths that would excessively lighten the hammer to get a very light double-action trigger pull weight on PPC guns. They were successful, but many of these finely-tuned competition guns came with special instructions to make sure they would work properly and reliably. Some were actually tuned to a certain brand of primer (Federal), and would not function reliably with other brands! Others had to be kept very clean and lubed regularly so the super-light hammer would move fast enough to hit the primer with the required force. The difference between these guns and a stock gun was often described as "slapping" the primer, versus "crushing" it.

In a K-frame S&W like your M66, I think removing the hammer spur still leaves plenty of mass to crush the primer, but reducing the mainspring pressure (loosening the strain screw) or using an aftermarket-brand mainspring in conjunction with a spurless hammer might cause problems. Also, if the gun is to be used for defense, I'd recommend trying it out with the EXACT ammunition you will carry in it, and I'd test it in the coldest conditions it might be subjected to, just to make sure the combo is reliable (cold temps can thicken some oils/lubes, slowing hammer-fall and causing misfires).

I've bobbed several of my K-frames over the years (and one N-frame), and never had any problems with them. The J-frames, on the other hand, can be touchy...

I've bobbed hammers on several K frames and one j frame and never had any problem with any of them. You do have to consider holsters as most thumb breaks won't work once you bob the hammer.

DJ is right on.Federal is the "softest" primer to indent,that's why they are recommended for lightly tuned revolvers.A stock gun isn't going to lose enough mass to make a difference.An old friend of mine that shot a K frame in revolver class pins and combat had it so light you could litterally use your pinky DA,but it was almost guarranteed one out of 50 wouldn't pop a Federal primer.