Hey David,
it is a synonym.
With 1.5 kW, you seem to have only spindles with ER-16 collets at maximum. With 2.2 kW and ER-20 collets, you can use tool shanks of up to 13 mm or 1/2".
Also with a 65 mm mount, the spindle does not clear the stepper which means you have to slid the spindle far downwards into the mount, increasing leverage forces. To avoid, one should always better buy the optional 80 mm mount. But I assume you know that already.
I would say this is not really risky. Don’t know with cheap chinese spindles with their flimsy hose clamps if they become loose and spread the cooling liquid around, but on the Mechatron you have Festo QS8 tool-less hose clamps. Never heard of problems with them. All you have to do is remove this cap and fill the cooling liquid in.
Generally with watercooling the cooling is independent of spindle speed whereas with air cooling the fan sits on top of the spindle axis and rotates with the speed of the axis, i.e. slower at slower speeds. However this has not the same importance as with the hand trim router, since spindles have no carbon-brush commutators that produce sparks and additional heat, and the current is always controlled by the VFD. If you ask me, I would always use water cooling since it means reliable cooling in all situations (and that’s what I bought, I own the Mechatron spindle and their cooling station)
Besides that, it’s mainly whether you want to avoid having to deal with coolant hoses, e.g. Michael @MikeH shows a air-cooled spindle. Better ask Michael in matters of air-cooled spindle.
In both cases, you have to set the lower speed limit in the VFD according to the spindle manual, since the spindle manufacturers do not recommend to run the spindles lower than the lower speed rating. Often the lower limit is 6,000 rpm but with some cheap chinese spindles I have seen that air-cooled models must not run lower than 8,000 rpm.