It's a trade-off. I would want the smallest one I could get by with, to give me the widest range of thread diameters I can do. But, small also means less stiff, which means slower cutting. The only one I currently own is a Maritool 0.372" diameter, probably the same one you're looking at. I've been using it for almost 18 months now. For anything smaller than 3/8" internal, I use taps. I typically cut at 6000 RPM/20IPM in 6061, and get threads that are absolutely beautiful. In most cases, I'll do two passes, rough to about 0.010" over/under, then a finishing pass at 35 IPM.

Regards,
Ray L.