Howdy Matt: Unless you're really doing critical work in color, while T coatings are helpful to ensure consistent color quality from one frame to another, they're not absolutely necessary in work that doesn't require such stringent controls. When Zeiss came out with the *T coatings about 20-25 years ago (yikes !) along with Hasselblad, they kind of touted the new multicoatings as a pretty big deal, one which sent some photographers racing for the dealer to trade in their older C lenses without the improvements. Yes, the coating reduces flare which results in slightly better color saturation, sometimes cleaner whites and purer blacks and also a reduction in ghosting. And, if you're backlighting a shot T lenses will help counteract light bouncing off the iris diaphragm producing a 5 pointed star effect. The other improvement in T's is that their shutter blades and diaphragm leaves have a dull black finish to prevent internal lens reflections.
As Eric pointed out, if you use a properly sized lens hood or pro shade, even shooting outdoors, C lenses perform admirably and unless you're really shooting strongly back-lit scenes, say a sunset, flare, glare, etc. should be minimized and T coatings sure aren't a guarantee against those problems anyway. I have both an older 80mm C and a T. IMHO, with a little warming filter and the proper shade, the C performs extremely well and I use it indoors quite a lot. I shoot with the T Planar out of convenience for filter sizes to my other T lenses.
One other thing to consider is that the C lenses accept a different size filter, like a 50 bayonet vs. the 60 bayonet for most of the T lenses (other than the 350 and 5oomm, as I recall). So, if you plan to get a 150mm Sonnar C, it might be worthwhile for you to get the 150mm *T if your filters, rings, and pro or other shades are the 60mm bayonet variety, or stepped down to the 60 bay.
David Odess, who's a factory trained Hassie tech with a lot of experience, has springs, iris blades, pc contacts, etc., available for refurbishing the older C lenses.
http://www.david-odess.com.
Take it light.
Mark