Hola Fran !
I have both, and both are very sharp, 150 it's true little faster than the 210. Both are very crisp in detail and very suitable for portrait, both, to get crisp image need to be used with tripod, as they are tele and even at 1/250s hand held 50 or 60 mpixel are very demanding (if you need perfection a 100%) .
If Your work is around people, the 150 or 100 are the lens to go, if You do landscape / nature etc. both are very good, it depends on what crop you prefer, as this lenses are tele but not so long (the 150 is equivalent to a 100mm or so).
Someone always say that 100 + 1,7x TC is the same as or better than HC150...
I test this combination and I can say this: maybe same as 150 but surely not the same as 150n in performance / sharpness.
If You choose the 150 try to find the "N" Version (the last developed) this one have better performance from the oldest 150.
Pay attention about the old 150 this was one of the first lens of H set , and it's easy to find very very old HC Lenses of this type, overused and prone to fail, search always the N and the one with sun shade with plastic ring (the one with metallic coupling ring are at least 10 year old)..
concluding, both 150 and 210 are capable of producing very good image, and the choice is always personal.
Happy new Year, Domenico.