Would I get the 3600 rpm spindle???
I don't know.
I am not very good at hypothetical questions.
The extra rpm can be useful as pit202 mentioned, for some steels but it is possible to compensate by using high rake cutters to get a good surface finish.
Are you getting the enclosed TL2-P or the open one? To be honest the idea of a chuck doing 3600 rpm just in front of my nose makes me nervous. Not a totally rational response I agree.