Like others, I can do a general TLR vs. SLR comparison, but not of the exact two models you mention. I have a Rolleiflex and a Bronica S2. I find myself reaching for the Bronica more, mainly because I can go wide with the 50mm lens. (I don't have the 40mm.) Generally I go super-heavy with the Bronica (bring the 75mm and the hulking 50mm, for flexibility; and sometimes a second film back), or I don't bring it at all. The middle course--taking the Bronica with only the 75mm and one film back (for example if I'm limited to a small camera bag)--makes less sense, and I do it less frequently. In that case, why not take the smaller, lighter Rolleiflex if I'll not be switching lenses anyway?
In the comparison above, I'm fortunate that I'm not giving up lens quality between the Bronica's Nikkor and the Rolleiflex's Planar, so weight/bulk is the main consideration. In your case, there is probably also a trade-off in optical quality to consider, since the Nikkor might outperform the Yashica's lens.
--Dave