Generally speaking I don’t like to poo poo advice…. Especially from other respected members.
However, Koolaid it may be, the numbers don’t lie. HSK63 has a higher bending moment capability than cat40, but I will say it is not much more, roughly 10%
Now along the lines of sir exkenna’s comments, both have their place, but the taper face contact of the HSK is superior to the CAT connection in high speed applications, but that doesn’t mean it doesn’t have equal to or greater capability than the CAT at lower speeds. That is likely more going to rely on the spindle bearing layout and torque capability, as both will likely be vastly different. Keep in mind higher speed spindle bearings have to harness more force due to inbalance and therefore should in theory be designed stiffer / stronger than the lower speed machines.
One thing that many don’t realize in the HSK format is you can convert these spindles to KM4X relatively easily as the ATC doesn’t have to change, Kennametal proprietary it may be, it has roughly a bending moment capability 6-7 times greater than that of CAT40, this means bigger cuts with longer tools or far heavier cuts at shorter projections can be achieved utilizing more of the horsepower that once was only usable with large drills.
just my 2c, take it for what it’s worth, not meaning to step on anyone toes or demean anyones opinion.
At the end of the day buy what you have budgeted for, can afford, and need, then run whatever you buy within its capabilities.